1994 Evinrude 40 won't run on both at low rpm

I bought the motor a year and a half ago from and older fellow who'd had since it was new. It had been serviced by professionals every year.
I ran just fine for 2-3 weeks until it grinded to a halt (while pulling a bananaboat). The shop found the bottom cylinder badly damaged. It was fixed and a new piston installed. When I got it back, it would run nicely for one minute or so, then the top cylinder died. After a lot of troubleshooting my conclusion was that the head gasket was leaking. The guy in the shop disagreed, but had to agree after removing the top.
The top was evened and a new gasket installed (free of charge).
But I still have problems. When I fire it up cold, it runs on both for a few minutes, then the top cylinder dies. When increasing power, the top cylider joins in at 2500 rpm. After running at high rpm for a while, then idling, it runs on both for a couple of minutes until the top cyl dies once more.
At full throttle, I get 5200 rpm. It's mounted on a Seabird 445 (14,5 ft) and gives 25 kts at max throttle. Which I think is a bit low.
The bottom cylinder compression is 145 psi, and the top one reads 130 psi.
Any suggestions or good advise?

Re: 1994 Evinrude 40 won't run on both at low rpm

Sounds like igntion issues to me. When was the last time the ignition system was looked at? Last plug change? Try swapping upper and lower plugs and see if problem switches cylinders. Hopefully you'll get lucky and it just be a faulty plug or something.

Re: 1994 Evinrude 40 won't run on both at low rpm

Thanks for answering.
I've replaced both plugs and also interchanged plug cables with no effect. It, could of course, be the ignition unit, but that's a bit expensive to replace without beeing sure.
Is it possible to interchange the carburettors on this motor?
My gut feeling is that the head gasket is still leaking. What are the symptoms to look for?

Re: 1994 Evinrude 40 won't run on both at low rpm

swap the coils completely and see if the problem follows the coil.