I have recently replaced my coolant reservoir due to old one having cracks. i have also replaced my thermastat. any time im on a uphill slope or going more than a 20 mile distant my 2002 taurus will overheat. coolant level is where it should be and there isnt any visible leaks. but coolant comes out of the cap. im not sure if its not holding pressure due to a blown gasket or what. there is no creamy substance on oil cap that insist its the gasket. the water pump is fine. im lost on figuring it out and dont have the means to put into the shop. can anyone help.

I would do the cheap trouble shoot first , have your radiator flushed and see if clogs.

There are several possibilities.

The thermostat is defective or installed upside down.
One of the hoses has collapsed.
The hose fitting on the new reservoir bottle is blocked by sprue or the hose is somehow obstructed - perhaps a hose clamp slipped and is pinching the hose shut instead of holding it onto the fitting - check both ends of the hose.

A leaky radiator cap would allow coolant to come out, but you wouldn't overheat until you ran low on coolant. In any case, the coolant should go to the reservoir before it comes out of the radiator cap, so look for a plumbing problem there first.