94 celica overheated now won't stay running under 1000rpms. Options

[iSMOKEtreez]

Ok I have a 1994 celica ST, with 289xxx miles. Supposedly at around the early 200xxx ish miles a engine with 150k miles was swaped in. Doesn't look like it tho.
The other day I was driving home it was about 70deg outside when it's normally been under 50deg everyday here in Missouri.
And I noticed the AC wasn't blowing as cold as it does, then she started to overheat she over heated and shut down doin 50mph lost everything, breaks, steering.
Got it home it's hard to start and now won't stay running under 1000k rpms. It has a nasty idle when she wants to stay running. And idk what it could be.

I just recently changed all this with in the last 1000miles.
Radiator, thermostat, fuel filter, spark plugs, spark wires, distributor cap and distributor spark thing. New air filter, did a radiator flush with the new used radiator in it. Also did a 3 part fuel system cleaning in it .

Any idea as to why she won't stay running?


***Edit***
I'm sure my collent went out the crack I have in the resivor because there's no cracks or leaks in the radiator.

***Edit2****
Also went from a synthetic blend to a full synthetic with in 300 miles

And I'm kinda thinking it's the head gasket.

This post has been edited by iSMOKEtreez: Feb 22, 2016 - 4:22 PM

[iSMOKEtreez]

She's still running decent. I droped the full syn and put a syn blend back in and she seems to run better. Like before the overheat. Well last overheat. The radiator cap didn't help she still leaks out of the cap but she's not losing much water over the days just burning them off.