250xs intermitent high idle surging loss of power.

[badger3x]

I have a 2005 mercury racing 250xs with 165 hours on it. It has a intermittent problem of a high idle 1200rpm surging of 100 to 200 rpm at cruising speed 3500 rpm and loss of power on take off. The problems almost always occur after the boat has been run to get up to temp then shut off for a period of time ( to fish approx. 45 minutes). Then when cranked problem will occur for the rest of the day. I have had the boat to 2 different mechanics with no avail. Both have taken it to the water and run fuel and air pressure test and it checked good, changed spark plugs twice they looked good both times. Changed tps sensor didn't fix the problem. Changed fuel filter all fuel line and replaced fuel bulb with a new mercury fuel bulb. I'm beginning to think it could be something in the cooling system like a bad sending unit or something sending false readings to ecm if that is possible. Boat doesn't have smartcraft gauge so I'm not sure of exact temp at time of problems. If anyone has had a something like this happen. I'm open to suggestions. Thanks for your help.