Engine problem - '85 Mercruiser 170

[scook]

Pertronix electronic ignition last year - starts and runs great (for a while).

Early June ran at approx. 3000 RPM for around 15-20 minutes, ran near idle for a few more minutes before anchoring up to fish - no problems. Maybe a half hour later tried to start up to move a little, idled very rough, wouldn't run up above idle (not in gear - no load). Tried a few times to start again over next hour or two, best it would do is very rough idle, wouldn't run up at all and died.

Yesterday, started with a little gas poured in carb (hadn't been run since Early June), idled and ran great, ran at approx. 3100 RPM for around 20 minutes and noticed RPM's very slowly decreasing, took it down to idle, it got rough and died almost immediately. At idle the oil pressure dropped to near zero - checked oil and it is good. Didn't try to restart - another long trip back on the outboard trolling motor.

Water temp in the river is around 70 degrees if that helps any.

It's got me stumped. Anyone have any ideas?