2001 Ford Taurus Coolant leakage

I have recently put $800.00 in car repairs replacing the collant resevoir, timing gaskett cover and water pump. I continue to have a problem with my coolant leaking from the cover. No one can seem to tell me what the issue is. I am needing to add a half gallon of coolant (minimum) per day. When I first drive my car in the morning (or after the car sits for a while), The needle tellilng the cars temperature will climb all the way to hot and then move back down to where it should be. When the needle reaches the top it will let a high pitch squeel sound and then I can see coolant leaking from the car. The car seems fine after this happens and coolant does not leak for the rest of my duration. Tasca states the pressure is building somewhere causing an air pocket and coolant to be pushed out from the cover, but has no idea why this is happening. They attempted to overheat my car and had no results - telling them that the fan is working. Do you have any suggestions?