Any help greatly appreciated.

Volvo 240. I'm going to give some history, as I am hoping it helps with diagnosis.

This past summer had the ac switched from r-12 to new. A couple of weeks later, when using AC in very hot weather, temp guage would go way up. Turned AC off and heater fan on, guage would go down almost as rapidly as it went up. Replaced clutch fan with used one twice.

Every once in a while I would check and see that coolant was a little low in resevoir. But, no leaks, no explanation.

In winter, on occassion it would take longer than usual to get the heater going. Thought it might all be linked to thermostat, so replaced that. Within about a month, drove the car across state, this time, temp guage went up even during the winter. (This may have happend prior to installing thermostat, can't really remember). The next morning, the car idled rough after start up. Checked coolant and it was low. More coolant added, drove car back across state. Coolant was low from time to time, and had the rough idle after start up, but increasingly even when engine was warm. (like after going into store for 5-10 minutes and coming back out to start up car).

One day, temp guage goes way up, and heater does not blow warm air. Check coolant, and it is almost all gone. This is where it gets very frustrating.

Fearing a blown head gasket (although no coolant in oil or oil in coolant) I take the car to my mechanic. I explain my fear of the blown head gasket and he assures me it is not. He replaces a hose below the resevoir and some spark plug wires and sends me on my way telling me it was fixed. Also said he checked compression etc. for blown head and no blown head gasket.

Drive 80 miles next day, on the way back the car overheats into red. Being a dummy I drive a couple of more miles with heater blowing (no hot air again) and pull into rest stop. I can now see water dripping under car, but don't know where from. Coolant is very very low. Fill up and get home. Now the car has the same problem as before, the rough start hot and cold. Finally get white smoke out exhaust. Take it into mechanic, guess what? Blown head gasket. Replaced head gasket as well as all other gaskets, and water pump. Also, sent the head out, check for warping, checked for cracks. Get car back, drive for a week, begin to notice strange rattling type sound from rear of car, and temp guage sometimes goes slightly above mid point.

Stopped in a drive through yesterday and the car goes into the red, like it did back only when AC was on and very hot outside. Problem is, it was neither hot, nor was the AC on. Check coolant, and it is down about a gallon. I've now taken it back to the shop (thinking of going somewhere else at this point mind you). By this time I am very angry as the problem that I originally took it in to get fixed still exists. They put a pressure test on the coolant system for 1 1/2 hour and the pressure did not drop. They ran the car for about an hour or so and can't find the leak.

Any help? Please? I think this shop needs all the help it can get in properly diagnosing this problem.