While trolling the other day I noticed that when trolling with my port motor my temp gauge will fluctuate considerably. On the ride out both temps will be rock solid at 170 degrees. When I start to troll, you will see the temp gauge go down to 100 degrees, stay there for about 10-15 seconds and then rise back up to about 165, then slowly make it back to 170. The engine never goes above 170 degrees. Just took the boat on a 10 day vacation and never had any swings in the temps of either motor, didn't do any trolling though. This gauge is also brand new, installed last season. Took a video to show you guys. Think it could be a thermostat sticking open and cooling the engine too much? Maybe a bad ground?
Motors are merc 5.7 efi with bravo 3 out drives, 2001.