weird heater/temp problems

Trying to fix a 2000 4.3 Blazer for a friend of mine. While I'm driving down the road with defrost on the temp gauge reads 210 then it will shoot up to 250 the heat coming out turns cold it stays like that for 30-40 secs then drops back down to 210 and heat starts coming back out of the vents. He had previously changed the t-stat. Just wondering if anyone else has had this problem. Thx

Install a new RAD CAP.

Yours is leaking and not keeping the 15psi needed in your cooling system.

Don't get an AC Delco, get one from your local parts store.

The Ac Delco is flawed from the factory and has a piss poor rubber fibrous gasket that fails.

I would lean more towards air in the system.

Yes, and air gets into the system through the faulty rad cap and when the engine is shut off and cooling down the rad will draw in air through the faulty seal in the cap instead of drawing in coolant from the resevoir.

Happened to me 2 months ago.