115 hp mercury optimax troubles

Bought a used 2010 115hp optimax tiller jet for my 18ft alumaweld stryker. Ran like a top for awhile then began to notice a slight misfire at high rpm’s.the problem gradually became worse with a loss of power from the starting point to 2 miles upriver.At that point I would be pegged on the tiller handle and doing half the speed I should’ve been. It’s been to two shops. One for 5 hours and one for over 10.Both mercury dealers and the last shop a very reputable dealer in the Portland oregon area.no one can figure it out. They have tested and retested everything under the sun and can’t tell me anything other than it’s a common misfire among all 3 cylinders. I recently took the pcm off and sent it down to Simon motor sports in California and it tested fine. At my Witt’s end.most of the mechanics I’ve talked to said to forget about it and buy a new Yamaha. I tested all the coils with my ohms meter all checked out good. Won’t even fire up now. So confused.If anyone out there knows anything about these lemons I can check please let me know. Thanks.

Jon... let's get your engine serial number.

Kindof hard to decide an engine that's at least 13 years old is suddenly a lemon... much more likely you've just not found the right dealer.

Suggestions:

-Compression Test
-Spark Test (including NEW IZFR5J Spark Plugs)
-Fuel QUALITY, Delivery, and Air/Fuel Pressure Tests (while cranking in this case, at 5/10/15 seconds of cranking time).