I'm not sure the 90 uses Honda's VTEC system, or not. If it does, then I have found that in certain RPM ranges, the ECM can't seem to make up its mind whether to use the high rpm setting or the low rpm setting, and will tend to hunt back and forth between the two, causing this surge, then backing off. On my BF225 that occurs between 3800 rpm and 4600 rpm - which is where a lot of crusing speeds take place. The surge moves the rpm range up about 150 to 180 rpm, then slowly backs down to where it started. See if that is your case - that the surging only takes place in a certain rpm range. I have found that the surge dampens out after about 5 to 10 minutes of constant throttle.
I'm not sure if there is a fix for this, or not. Next time I get my motor to the dealer, I plan to ask them to try a new ECM to see if that fixes the problem.