Volvo: Volvo still overheating although all the work I thought I needed has been completed, radiator cap, head gasket

Question
Had a problem with my '00 s70 turbo overheating a couple weeks ago. Brought it in and was told I needed at the very least a new water pump and timing belt, and would probably also need new radiator hoses, new thermostat and seal. I had them put in the new water pump and timing belt and then returned to the shop a week later for the rest when the problem came back. Everything was fine for a day or so, but this evening the temperature gauge started acting up again. It would fluctuate between 50% (normal) and 75%, but eventually it went up to the first red line and stayed there til I finally got it home a couple minutes later. Definitely some mild hissing; I am pretty sure this was the coolant boiling. I've been told by several friends that the problem might be a faulty thermostat or perhaps a bad radiator cap letting the coolant boil too low. Any other ideas on what it could be? The mechanic told me the radiator was fine. Any input greatly appreciated. Thanks!

Answer
Stefan,
The t/stats in those cass fail often and could be the problem. Hopefully you did not overheat the engine too much where there is addition damage. Have a new cap installed as a precaution.
When these engines are overheated the head gasket fails or the cylinder head warps. Be careful not to overheat.