25 hp 1990 Evinrude surges

[Qkjmpa25]

I just bought a used 1990 Evinrude 25 hp E25TELESB.
It starts great, idles fine, all other speeds are great. After trolling for a while, when I accellerate it surges high to low, high to low. If I fast idle for a few minutes and completely close the throttle, it will then accelerate normally to high speed and maintain that speed. I replace purge bulb but no difference. Pumping bulb also made no difference. My guess it us over heating.
I asked a mechanic because at low idle there is no cooling water stream, yet at a higher idle it has a good stream. PO said he had replaced the water impeller recently. Mechanic said at low idle they don't produce a stream, water only exits through prop.

[Solarman]

I've never had a motor that didn't pee at all speeds, yes lower flow at idle, but still a stream. Think I'd call a few different shops and check that? Oh, also water has to go to the powerhead before going back to prop, the pump sends water up to exhaust, pee hole, and thermostat at one time.

I've heard of similar surging on other motors after extended idling/ trolling type running. And have heard several thoughts? basically (not sure I believe it all, but no other answers that fit yet?), temps of motor warm up (lower water flow rate) combines with lower fuel flow (slow speed) allows fuel in float bowls to vaporize- may even cause a slight back pressure on low press fuel pumping- anyway, it's similar to a partial vapor lock scenario. However, seems in almost every case, going to a bit faster idle (water flow increases, fuel usage pulls more fuel flow, then everything seems fine again?

I still think something is weak, but have heard same thing on some new motors?

Anyway, good luck.