what would cause my 2005 chevy equinox to blow the coolant back out the resivore

Possible head gasket leak.

but i just had the heads worked and done a pressure check on them new head gaskets and all

If the water pump impeller fins are worn and the coolant isn't circulating properly it could boil right out of the overflow. You didn't mention overheating but I thought id throw that out there anyhow.

If this is a compression leak and you eliminated the heads then the block is the remaining suspect. If the head gaskets were installed incorrectly or the heads torqued improperly it could also cause a compression leak.

Was the engine overheated really badly?

but its not all the time its just when you are in the gas above 2000 rpms in a 30 to 45 seconds

A heavier load on the engine causes higher cylinder pressures. I would do a compression test and maybe a leak down test but my guess is poor workmanship when the heads were installed.