Hasn't been getting real warm yet but will soon. Just on short errand runs after the car warms up it will get up to 260* real quick then cool down real quick. What is the deal? Over the winter the vaccum line going to the heater diverter valve broke so I spliced it with vaccum hose. Could the diverter valve cause this? Or could it be the thermostat?

Could be a stuck thermostat that eventually opens now & then. I had a problem like that before. With a stuck thermostat the temp just climbs & climbs really fast.

i had the same problem, get hot very fast, then maybe it will cool down but eventually get hot again.

it was my t-stat.

On another note, it is possible to get defective "brand new" thermostats too!! When I first tried to replace the t-stat The same problem was present. A rapid climb of the temp. On a hunch, I got another t-stat & problem was solved! The defective t-stat was a Stant one & I got it from Checkers. I have such bad luck with parts from them. It's almost like they get the bad bunch of products from companies. Got the good t-stat from NAPA. Same brand.

Alright, thanks guys for the info. I'll try and get that thing replaced with a 170* sometime soon.