Blackberries

I'm not familiar with BMWs, but it sounds as though it may be a thermostat issue. They just wear out over time. In many cars they are very easy to change. Just remove one of the big coolant lines (when cold of course, with the battery disconnected) pull the old one out of the hose, disconnect some wires, put the new one in, reconnect wires.

Also, stock engine temp gauges aren't incredibly accurate. They are "narrow band" in that they really measure just hot/cold as in on/off. In the middle doesn't really describe much (just gives the driver a warm fuzzy feeling), but should be ok. Usually its when the needle is 3/4 or above that something has gone wrong, and it is usually too late to respond.