I have been chasing a misfire on my 2001 DHS with 102K miles for some time. Initially I suspected a vacuum leak or plenum duct tear, not the problem.
Occasionally it would give a code P300 (misfire) and a couple of times I got the flashing SES light for a few seconds.

My AutoXray scanner indicated the # 5 cylinder (rear bank) was causing the misfire code. Sometimes the count would be quite high and other times it was minimal. I swapped the #5 injector with the #2 injector to eliminate the easy and less expensive fix, not so, the misfire was still on #5 cylinder.

That left the coil module as the fault. Ordered a Delphi replacement from RockAuto for $170. installed it today and misfire is gone and no codes. Of course it would have been much easier if it had been a front bank coil module.