Could be the sensor
I just bought a 97 528i and asked him if it had over heating problems... He said yes and had all the records of the work done. Looking the at the dealer receipts the last thing that was change (on the third visit for the overheating) turned out to be the temp sensor. I live in one of the hotest areas in the SF bay area, my car never hits halfway, even in 1 hour of stop n go traffic.
Hope this helps...