The coolant temp on my 1994 5.7L Silverado varies widely. When I first begin to drive it gradually rises up to 210° then suddenly drops to about 150°. I changed the thermostat (old one was 195° too) but no change. I've also topped off the coolant several times....about a pint each time. Heater return hose is hot. It's obvious that the thermostat is doing its job but why does it go so low? Is this common? If not what is the cause?