Fluctuating temperature?

[detzy]

1992 5.7L Mercruiser raw water cooled. new outdrive last year (although this problem was happening prior to that)

My temp gauge runs up to to about 150 at idle (its a 140 T-stat) but will then runs very erratic at high RPMs. It will sometimes run up to 200 and back down to 140 in a matter of 2 or 3 seconds. Today running steady at 3200 RPMs temp moved from 130 to 190 and back within about 30 seconds. I have checked the risers at full thottle and they are only slightly warm to touch so I do not think the engine is overheating. Is that a correct assumption? If so do I have an electical problem?
I checked and tightened all connection on the temp gauge. T-stat and housing are new this year. How likely is it that temperature sender in the t-stat housing is bad? The part is $50 so I hate buying one on the "hope" that is my problem. I know they work off of some resistence process but not sure if they are likley to (or can) fail.

Any advice will be appreciated.

what happened i have sane issue i have one engine overheating goes to 200 then drops back to 140 so i replaced thermostat maybe helped a little but gauge is still erratic bounces up then back down prognosis please

 

[Bondo]

Ayuh,...... Welcome Aboard,..... Tell us about yer motor, rather than Mike's, 'n we'll help ya fix your's,....

 

[dubs283]

With any raw water/open cooled mercruiser cooling system issue the first item to inspect is the sea water pump/impeller and associated hoses/passages for blockage