87 300SDL no heater above 2000rpm

[banshee350]

I just bought this car heat worked great driving around on side streets ,when I was test driving it.on the way home I noticed that when ever the rpm is above about 2000 rpm the heater" actualy the defroster thats a different problem may be, air only comes out the defroster vents no matter what button is pushed eccept for the one with the o then it shuts every thing off" back to the heat above 2000 air temp goes cold,let off the throttle and it returns.Also found that the rad cap is bad did not notice it before but it is missing the seal and relief spring that seals in the presure in the sytem could the pump be cavatating due to no presure.I had removed the cap and smelled the over flow bottle to see if it had any exh smell and it does not.Car runs great has lots of power temp gauge ran at 80c or just a little above the whole time although it is cold out 40 to 45 deg.Also any one know what these motors are governed at what should be rpm at 65mph.One other question there is a bypass valve of some kind in the exh that is between the in take and the exh there is a pot on it with a little pipe that looks like a vacuum line could go onto, and what looks like a adjutment screw is this a egr valve or a manifold boost relief.