My bet is on an air bubble. Just been through this, but I could get heat if I did some hard pulls. 5MT SS here.

Ok guys! I replaced the t-stat and coolant last Saturday, a week ago. Everything went smoothly. Just yesterday it started doing the same exact thing. It went a whole week nice and hot. I was on my way home from work last night, started hot and then again went lukewarm. Now I notice once it goes lukewarm, it tends to blow more air out of the vents, then once it goes back to hot, it slows down a bit, like it has a fluctuation in a door or something. My mind is boggled. What I don't get is, why it went a whole week without an issue. I would think the issue would still be daily as it was before i changed the t-stat. I did noticed that the seal in the t-stat didn't look good, so I thougt I nailed it.

Did you burp the system real good? Sometimes those air pockets can be real stubborn. Other than that, you're back to a blend door problem.

Dammit, lol! Yes I did burp it well. It was an SOB!! Is it one particular blend door?

Check the link in post #20.

Did you check the heater core like donbrew suggested?And here's the link for the blend door again.

Yes, blew air through it

Did you use a AC Delco thermostat ? Dexcool ?
What are your temps running now ? Are the temps the same when it was working correctly and then started acting up again this time ?

There's pictures in post #2 here...

https://www.chevyhhr.net/forums/prob...ctuator-51029/

Yup, delco t-stat and dexcool. The temp currently is running between 183 - 198. I believe the temps before were in the 190's to 200