My temp gauge in my 1988 4runner (3.0V6, SR5) started acting up.

It was working ok, seemed like it always sat at 1/2 way after just a few mins of driving. I had to replace the speedo cable so while I had it out I thought I'd check out the wires on the temp gauge (as done in definitive temp gauge fix sticky). Didnt find any broken wires but did see some corrosion starting and a small glob of solder on the copper wires themselves. Cleaned off the corrosion and removed the solder (seemed like it didn't belong thought it was just a drip from assembly).

Now when I start the truck it reads cold like it should. But after only 1 mile (maybe 1-2 mins) of driving it goes from cold all the way up to the bottom of the red zone. Never goes higher than that. If I pull the plug off at the sender it drops back down to the bottom. I checked the engine temp with my infared tester and am seeing 180-190 at the block and T-stat housing so don't think its acutally running hot. The upper rad house and radiator are 170ish.

Could the sender be bad or did I screw up by removing that tiny drip of solder?