How do I make my 25HP 1988 Johnson outboard motor run smooth in mid range?

My motor starts great, idles just fine, trolling speed, no problem, wide open runs great but when I hit the throttle is stammers a bit ( 2 or 3 seconds or more ) before it smooths out and takes off. I have changed plugs, gapped them at 30 thousand and have the factory recommended plugs in it. Also tried a tank of fresh high octane gas ( no ethanol in it ) and mixed oil carefully at 50 to 1. I stored my boat last year and drained it for the winter with no ethanol and Sea foam stabilizer was ran through it before running it dry and storing it. I have check tanks for any junk and there super clean. I can't think it would be dirt as it will run good top speed for long periods without a miss. Just at take off when I hit it. That is about all I can tell you and hope someone has a good answer. Thanks