Pontiac: PLEASE HELP-UNUSUAL SUNBIRD PROBLEM

CBConsulting wrote: 

Sounds like air in the system.
When a batch of hot water hits the temp sender, it goes up.
When there is no water on it, it goes down.
Get the air out. There should be bleeders.
Do it quickly or the engine could be ruined.

Hi there, not sure if his is related, I am the owner of a blue 93 Sunbird and took the car to Canadian Tire in Dartmouth NS on Jan 9/04 exact same issue you noted here, are you a tech talking about my car or looking for personal use, anyway, I have replaced the heater core 2 times in the last week, the dealer shop told me the Cn Tire one was not good, they were wrong, at this point I have some fool that spent according to him 11 hours working on my car only doing the same exact tests I told them I already paid for or Jan 12 when I picked the car up...Forbes Cev in Dartmouth says they want me to pay $947.01 for telling me they don't know what the problem is, as you can imagine I am enraged, I mean how could I spend a thousand bucks to diagnose nothing but a "we don't know what it is" andespecially since I gave a detailed history of the work already done. Can you possibly shed any light on this situation

things done at can tire
Jan 2/04
antifreeze flush and fill

Jan 9-12/04
test for hydrocarbons in antifreeze (71 in fluid & 81 open air, usually a head gasket will give about 1000 or more on this test)
no oil in antifreeze
no antifreeze in the oil
no white antifreeze residue on spark plugs

new heater core
thermostat
take water pump out inspect and replace
flushed system twice again

Forbes chev
Jan 21-22/04 (after I told them about what was done already at CT only a couple of days ago...)
forward flush
reverse flush heater core
replace CT heater core
install GM core
test CT heater core (no issues)
replace CT core and remove GM core
flusehed system again
took water pump out, inspected and re-installed

Still no ideas, only that air might be getting in somewhere...