On windy (5-6) or windyish (4-5) two or three sail reaches, particularly in choppy water, I have capsized without really understanding what has caused it.   It has not simply been a case of being overpowered and capsizing to leeward, but rather a series of oscillations.   Either promoted by a lull in the wind, or the choppy water, I think I attempt to level the boat by steering.  As the hull heels to windward I probably overdo the steering promting the hull to start screwing up to windward and needing more unhelpful steering to counter it.   All too often after one or two oscillations a capsize results.

Club maestros suggest I am steering too much and should be compensating for gust and lull by dumping and sheeting mainsheet in response, and with weight movement if necessary.  My super crew has evolved with a super helm and has never needed to look at what the stick man was doing, the super helm in question is not sure what he does himself either, he just does it.

If Merlin pundits can recognise the symtoms, and offer helpful advice towards an effective cure, both I and my suffering crew would be grateful!  Its getting repetative and colder.

Chris,  I have this experience too.  I noticed it when I went from my old echo design to the much newer, wider, flatter NSM4.
When the boat gets up onto the plane it becomes considerably less stable.  This is due, I think, in part to there being far less of the boat in the water and the increased effect of steering due to speed (i.e. the faster you go, the more your foils work).

You should definately be using your weight to control steering at speed and always using as little rudder as possible in all conditions.

I watched Glen Truswell and Paul Davis flying along under 3 sails at Blithfield and was amazed at how much work they were both doing with the sheets.

It's only the last couple of times that we've been out on the boat in wind and actually been able to get the boat planing and hold it for more than a few seconds so I think it's just a case of practise and accepting that sailing fast off the wind in a Merlin is a difficult thing to do but possibly the most rewarding part of sailing there is.  Our next step is to become better at moving around in the boat when we're planing because we end up dumping instead of hiking.  If we can't sort it over the winter there's always May training to look forward too!

Try to immobilise both ankles, this will give you equilibrium in the lower half of your body preventing you "screwing up" in the boat. Try to be more scared, this normally hieghtens the senses leading to an increase in awareness of what the boat is doing and keeps the andrenaline pumping leading you to react quicker. Listen to more Little Richard, this will give a better appreciation of Rock N Roll and how to deal with it, keep going splosh! It makes me laugh

Get your weight well back and keep the boat dead level.  It's a balancing act & you are steering the boat to keep under the rig.  You HAVE to be able to bear away slightly in a gust & if you are a little bit heeled you will stall out the rudder without the boat responding.  Not that I've had any practice at it this year!  The sails are a much better way of steering the boat than the rudder - if you are sawing away at the rudder, calm down, sit down & enjoy!  You depower the main on a reach by pulling on lots of cunningham but EASING the kicker to increase twist - not right off though as that will make you roll more.  By your sail number you have an NSM of some sort I expect & they aren't as easy in these conditions as a wider, flatter boat.  I actually think the boat gains stability on the plane, the loads on the rig drop as you accelerate and the rudder has more bite

I find the best thing to do in this wind even single handed with 3 sails is you get your weight out and aft as the boat speed increases whilst playing the kite and main with the jib fixed and be subtle in the bear away on the gusts, no big grabs a the helm.
Also have a look at you tiller and extension these should be long enough to give you plenty of scope for comfortable control in the full hiking position.
Also worth discussing is how much board to use? This may case some debate as I have sailed with those who keep there boards down come what may and others that almost fully retract on reaches & runs.
I work in the middle and aim for no less than 1/3 down and sometimes more in a big sea motion.
But finally I advocate the fear factor as stated above I have often found myself wondering how on earth to get the kite down!!! But thoroughly enjoying the ride.

Also check that you have sufficient friction to keep your centreboard in the half down position - a wipeout to windward is guaranteed if the board pops up on a power reach!  Better still, my boat has a rope from the top of the board to a cleat on the case, which on breezy days is set so that the board cannot be raised above half. 

Whilst on foils, check that your rudder can't move either.  Just a small bit of aft rake, or flex in the stock, will make the boat much harder to control downwind in a blow.

A bloke at our club has the same problem but applying aerodynamic principles we found that on windy days the vortex's off of his eyebrows was causing an unstable air flow on the back of his main causing fluttering and instability. The solution was simple, he shaved his eyebrows!