88 Blazer 4.3. I changed everything on mine (Cooling system) and it is doing the up and down temp. This is making me crazy because I am pretty good at working on my own stuff.

 

Sounds like air in the engine coolant. With a cold engine raise the front end with the passenger side slightly higher, radiator cap off, and idle until it reaches operating temperature. Watch for bubbles being burped out of the radiator neck.

 

And expansion tank needs to be filled to the mark, else it may pull air back into the system.