I'm in a similar situation. Head gasket blew in November on my '97 Outback. Had it repaired as well as a new theromostat installed. Ran fine for a couple months, but now is exhibiting similar symptoms to your vehicle. Overheating and at times, the gauge will spike and then return to more normal levels. Very inconsistant. I had a compression test done at a garage and it passed. So now, I'm suspicous that the radiator may be bad and there could be corrosion that is ihibiting the flow of fluid.
Perhaps getting a compression test could help you isolate the issue. I'm in the market for a radiator, so if people know where a good source for one may be... Let me know.
Tim