I have replaced the thermostat twice in the last two years on this beast, but still the heater blows only lukewarm air

I am using OEM thermostats, 192 degree.
She only climbs about a third up on the temp guage. I flushed the coolant, but did not help.Not even a little cardboard in front of the radiator helps much, and I hate to go that route.
Is it possible the radiator cap is faulty? There does not seem to be much pressure at the cap, even right after I shut the car off.I can basically just turn it right off and there is very little hissing. With my other vehicles I have to wait a while to pop the cap off.
Any ideas or thoughts would be appreciated!!