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When the speed drops, it sounds like it's running on one cylinder. Are the exhaust pipes about the same temperature? If you can keep it running the slow speed for a while and check, that might give a clue.
Are the valve clearances in spec? A compression test would show low pressure if they're not closing all the way when cold. Checking the clearances might be just as easy as the pressure test though.
When I got my 250 the valve clearances were too tight, and when the engine was cold it wanted to either idle way too fast or hardly run at all. I couldn't set the idle RPM to anything stable.
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