I just installed 180 degree therms on my flathead yesterday and now it heats up good (there weren't any before) but the right side seems to jump around a lot. The left side of the motor heats up to almost 200 and then drops to 180. It stays right around that area. The right side will heat up to 210 - 220 before it drops to 180. It doesn't stay at 180 though. It builds to 200 and then drops back to 180. It does it over and over. Why would it do that while the left side doesn't?

 

Here is an excellent article I saved from the flathead site -- applies to all thermostats tho. The ones you have are not matched to your engine.