You state it's spark, but are you sure? If it dies slowly, that sounds more like fuel related. Sparks are normally either there or not, not gradual. When it won't start, pull a plug, is it black or even wet with fuel? Then you have no spark, if it's very clean, it's a fuel issue. If plug reading is not something you're used to
( http://www.tuningmadeeasy.com/.../spark-plug-reading/ ) then the ultimate test is to take out a plug, put it back in the plug wire booth, lay it on the engine so it touches ground, crank the engine, then you should be plenty of sparks if it works.
Assuming it's a spark issue, it's probably electronic components getting too hot. While it would be nice to be able to tell you which one, my experience (not being electrical engineer you ask for) it's best to simply change components one at a time until the problem goes away. Not buying everything, but old coils etc, borrow from friends, if you have ignition box, take it off and rewire. Tedious, but works...
My $0.02 though is that you have a fuel problem, unless you've tested and found otherwise. Too rich, maybe a broken or hardened power valve, way too rich at idle, and in the end the excess fuel overcomes the spark on the plugs. Check the plugs...
Or even a vacuum leak, engines idling and then dying slowly can be vacuum leak (
http://www.tuningmadeeasy.com/...e-the-wonder-tool-2/ )
Just guessing from a distance...
