Marine: Boating, Sailing, PWC, GPS and Navigation - '82 Johnson 35hp outboard running problem

I have an old 1982 Johnson outboard 35hp with the long shaft and electric starter. When in gear, about every 20-30 seconds the engine backfires or a gear slips. The whole motor jerks upward an inch or so and then resumes as normal for another 20-30 seconds. This happens only when at above idle speeds. The motor has been doing this for a couple years; however, we seldom use it due to this problem. I recently changed the impeller for a cooling problem when at idle. I am worried to take the boat out any more because of the possibility that it might just give up in the middle of the lake. Any help or ideas are appreciated. Don't hesitate to ask for more information. Thanks.

It sounds like a mis-fire. Now to list the possible causes. Bad fuel(water in the gas). mis-timing, bad plugs,leaky head gasket, cracked head. Dose this happen at a certain rpm? Can you duplicate it with without being on the lake (using mouse ears)?

I know for sure that it's not the plugs or the gas. Sorry for the slow reply, my father does not seem very motivated to fix this (he's been trying different solutions for several years). I am finally trying to do this for him. I would like to run with bunny ears, but I have a few questions first. I remember running it last year with them, but we never shifted into gear. If I recall it only happened when in gear no matter what the rpm. I have heard that it's possible to put a garbage can or something around the engine and fill it with water. Then start and run in gear. Would this work to *safely* test engine in gear, under load? How much power would I be able to give it when running in a garbage can?
Secondly, once I do this, what steps would I have to go through to winterize it? Sorry if these questions are obvious, but I am trying to get this solved and my dad is slowing me down.
A guy on another forum said this:
"Mike it appears you may have forward gear clutch problem. By explanation of the problem a replaced clutch will more than likely cure the jerking while in gear and under some power."
Would you think this is correct? If so is there a way to check for sure if this is the problem before I go and buy a new clutch?
Thanks.

Running a motor in a garbage can is possible, however, do it outside. You will have water everywhere and oil mixed in with the water. You can safely run it in gear but make sure that you keep water running to the can, so you don't run low, as I said it will splash out when in gear. You may want to remove the prop prior to doing this test.
To re winterize it you just have to fogg the engin and check for water in the lower unit. Speaking of the lower unit, that is where the clutch is. I would not take a lower unit apart with out a repair manual on hand!!!!!!!
If the motor runs good without being in gear and has trouble in gear, the problem is probaly in the lower unit. Check for metal specks in the gear oil.
Some times you can get a deal on a lower unit on e-bay. I bought a head for a 35 johson form a guy that had 3 or 4 of them, he may still have parts. If you need his name let me know and I will let you have his e-mail.
Talk to your dad as he may be willing to assist you.