I have a 1982 90 hp Johnson Seahorse on an 18' Privateer. The motor will run fine at idle for awhile, then after 5 minutes or so it will drop rpms and barely run or stall out, especially when put in gear. If you increase the throttle it will run rough and vibrate a lot, then all of a sudden it will increase rpms and run smooth. The motor runs fine at high speed, but it will speed up suddenly when increasing the throttle to planing speed. After testing the spark on all cylinders, it appears that one bank (2 cylinders) is intermittently losing spark. I think this is a problem with the powerpack, but I'm not sure if there are any other electrical components that could cause both cylinders to not fire at the same time. One time when running at planing speed the motor slowed down all of a sudden by about 7-8 mph (no tachometer), so I slowed down, then sped back up and the motor ran fine. Because of the way the problem suddenly starts and then clears up, I'm almost positive its something with the ignition, but its hard to test because it doesn't happen all the time. Thanks for any ideas.
Sounds as if it could be an ignition problem, as they tend to occur when the engine is warmed up and the ignition componets get to operating temperatures. Your 82 has two power packs, one for each head. If you are losing spark on one side of the engine, swap the power pack with the other one on the other side. If the spark problem moves with the packs, one pack is defective and should be replaced.