Re: neutral safety switch operation
I don't believe there was an '80 90hp, but at any rate, that remote is probably very similar to my '79. Your problem could also be a key switch that is starting to fail, and not making contact between the B and S terminals on the switch. A continuity check at the plug wouldn't tell you which one (key or neutral) was causing the problem, so you'd probably have to open the remote to locate the problem. If you do remove the neutral switch, be careful with the detent spring just below it (very strong spring).
Since the problem is intermittent, it could be difficult to isolate. The next time that it happens, try moving your lever forward and reverse a few times to allow the neutral switch to open and close, then try to start it. If that doesn't work, jiggle your key a bit as you are turning it. If either of those work, it'll help you narrow down which one is causing the problem. If neither work, and you don't want to pull start the motor, just jump 12v from the large battery terminal to the small ignition terminal on the solenoid (key on).
I'm including a diagram and pic that will hopefully help you in locating the problem.
Edit: Didn't see F_R's and Chris1956's posts until after posting mine. Listen to what they say.