E-Tech 250 won't rev above idle in Forward

[JimJamie]

This has happened twice now. The first time was last summer. The warning system beeps, the engine dogs. We limped home on what sounded like 3 cylinders. By the time the mechanic looked at it there was nothing wrong.
Same thing last weekend after an hour of ocean cruising. We nursed it five miles back to the dock. Once tied up the engine ran like a charm, and the same when we tested it the next day.
What is wrong?

[boats.net]

Have it checked with computer diagnostic to see what fault codes are stored, then will be able to run diagnostic to find problem.