[shorod]

Since the temp gauge drops when moving down the road, I don't think the thermostat is the issue. Likewise I think you would likely have boil over issues from a headgasket failure when running down the road. If a headgasket were leaking into the combustion chamber you would likely be able to smell it in the exhaust and get bubbles/boil over in the reservoir, like davebigs mentioned. If it was leaking into the oiling system, your oil on the dipstick would look like chocolate milk. If it was leaking to the outside world, you'd get steam with a sweet smell. Any of these would lead to a drop in the level in the reservoir, assuming the system is full enough to continue to pull from the reservoir. That gets back to the question of the level in the reservoir changing when cold versus hot. If the level is not changing, then there is still a chance you have air in the system and it is not able to pull from the reservoir. Does your Cavalier have the pressure cap on the reservoir, or is there a pressure cap on the cooling system and a separate cap on the reservoir?

-Rod