I recently tried a raised pressure cap on my cooling system. It went fine for about 20 miles, then it started to act up (it only cools at idle with the cap off) I replaced my thermostat, took off my radiator, cleaned it out, my compression is 150 across the board and my system will hold 16psi for at least 20 minutes. I'm also not getting any heat from my heater core and I blew through it and it is also not blocked. My water pump is not leaking and has no shaft play. Any thoughts similar problems or diagnosis would be appreciated. I've done some searching and have found nothing similar.