might check the voltage coming from your alternator, could be a faulty voltage regulator? check your main loom ground near the battery. make sure there's a good connection, clean off any corrosion... ive never come across this problem but thats what i might check first...
doubt it'd be a voltage problem, you'd have problems with more than the coils. are they burning out on the primary or secondary? Is it the same coil each time or different ones? I'd be removing all the coils and make sure the earth point and bolt are clean, no corrosion etc etc. (the bolt retaining the coils other side to the posts) How old are the leads? if they're old and not transferring the spark to the plugs correctly could cause issues. How old are the spark plugs? are they gaped correctly, incorrect gap will cause the coils to output a larger voltage to jump the gap and put excessive load on them. I don't think it'd be the dfi module, a faulty output transistor inside would be more inclined to fail rather than continually burn out coils.
