[vettestr]

The group has already given some great hints and common theme is coming out as an electrical/electronic fault and I also agree. An easy ignition test to help nail down fault would be checking the ignition at the plug. Remove a plug wire and crank engine looking at spark at the plug (this should be a neon blue color) a yellow or orange spark is a weak low voltage result showing need to replace coil, condenser or last the box. Check when cold so you can compare if it fails when hot.
Also watch your volt meter because MSD units hate low voltage and will faultier at low supply voltages (low is anything below 12.0 VDC) Ignition system could be fine but drop out after battery V drops from fans and running on the battery if charge system is funky but easy to check the gage. An engine running should show about 13.5 to 14.5VDC at 2K rpm. Hope you find it soon so you do not have to worry anymore. Jeff C