I share your pain. My 97 528i does similar things. It started w/ an exploding expansion tank, then I replaced the thermostat, fan clutch, and so on. I'm $1-2K into this and it still shows signs of overheating, mainly at idle. I can watch the needle climb into the red and the light comes on. I rev the engine and it drops down to normal. The next day I'm making the same drive, same time of day, same outside temp, same traffic light, and no signs of overheating. I'm not a mechanic by any means, so I'm not giving you mechanical advice. I'm just letting you know I'm farther down this road than you are and still haven't figured out what's wrong. The previous replies offer good suggestions, but make sure $ you spend is worth it. I replaced many parts because some mechanic assumed they were defective, not because they actually were. By the way, my issue started 3-4 years ago. My saga is recorded on this forum a few years back if you want to search for it.