76 merc 900 runs rough

by Steve
(Prospect Hill , NC)

I have a 90 hp merc 1976. was running great for several days and then would beging to run very rough at all trottle positions (even idle). Starts fine, runs good to begin with(maybe five minutes). It will then start to run very rough and will not accelerate. has stalled out a few times when trying to acceleraate. I've checked all wire connections and all seems to be okay.
Now for the crazy part, while running rough i can slightly turn the ignition key or push either the up or down button on the tilt/trim and it will run fine again for a few seconds. If I go WOT and do either of those thngs it will acclerate and run great at WOT. It will also idle fine if I do either.
I was told that this was an issue before I got the motor and it was repaired at a boat shop by switching the positive and negative on the tilt/trim. I was not ran oafter this repair until I got it. I've already tried swiching the wires again and it made not change.
Any advice is greatly apprieciated.
Steve