2003 Honda Civic 2003 Honda Civic overheating at lower RPM

I have a 2003 Honda Civic that was in an accident recently. I was hit on the driver side forward of the front wheel. Insurance paid for the repairs and the radiator was one of the things replaced. The car ran fine for about 2 weeks, then started to have overheating problems. I called the mechanic that did the work and arranged to have him look at it again. That night I checked the coolant level and found that there didn't appear to be any in the expansion tank. I refilled the expansion tank, and the car ran fine. The mechanic couldn't find any leaks, and the car wasn't overheating anymore. A week later, the car started overheating again, and the expansion tank was full. The car would maintain temp as long as I kept RPM over 3k. If I went up a hill, or if I let RPM drop it would start to overheat again. The mechanic told me he thought it might be the heads (and he didn't do that kind of work) or the water pump/belts. He also told me that it wasn't caused by the accident, so insurance wouldn't cover it. I took it to my mechanic, and he looked at it. He couldn't get it to overheat sitting in his bay, and didn't really see the problem driving it around a little. He did replace the water pump, timing belt (getting close anyways), thermostat, and some seals. He checked the coolant for oil twice, and then once when he did the pump replacement and didn't find any. The car ran fine for another week. Yesterday, the car overheated again. I opened the hood and found the expansion tank overfull and coolant spattered on the fan and the surrounding area. I got it home after letting it sit for an hour. The expansion tank was still full. Three Hours later the expansion tank was still full, and the radiator was under pressure (air escaped when I opened the cap). After looking online at people with similar problems, they recomended checking for blockages in the radiator/heater core, or replacing the radiator cap. I have the mechanic getting a radiator cap, but is there something else we are missing? My mechanic is as baffled as I am, he doesn't think there is a blockage, as he was able to push water through fine.
Thanks.