Engine Why would the coolant reservoir cap blow off???

Why would the coolant reservoir cap blow off???

This is concerning my 91 mr2 turbo.

My overheating is under control... most of the time. However, on long hour+ drives(trying to see if my car can last all the way to Topeka for NA2005), the temp needle will rise pass the midpoint and go into the redzone. I then see white smoke(coolant)coming from the engine. Pull over, pop open the lid(hood), and see that the reservoir cap(NOT radiator cap)has blown off the bottle. The coolant is smoking from within the bottle. WTF??? Oh yeah, I lose heat at times too... the heater start to blow cool air when the cap is blown off.

Already changed thermostat, removed radiator and had it cleaned(it was clogged), and still this!!! Waterpump and headgasket are not very easy and/or fun jobs on this car so I hope it's not either. Waterpump should be ok because coolant is moving through the system.

It's probably the radiator cap. It sounds like it's allowing the pressure to go right by...

I tried two different OEM radiator caps and I still get the same thing. One was brand new, too! Maybe I need to 'test' the radiator caps.

Yeah you could try it... other than that I'm not sure.