starts and runs rough, however after 20 minutes loses power and wants to stall

Have the ignition module tested. If bad replace the module and the stator at the same time.

Agree with #1, Also in another direction, take a fuel sample and check for water. Water will settle and be picked up again while driving. Check engine oil for gas mix and over full from a leaking fuel pressure regulator. (possible stuff) Good mechanic and an old fashion oscilloscope will be useful on this engine!!

My chevy had clogged catalytic converter when that happened to me, I was getting oil pass the weakened rings, because my engine got hot when the radiater had an ice blockage.