It has to be your water pump.
When was the last time you had the radiator flushed?
Stupid question but if you haven't done it in forever that could be a major part of the problem.
Also radiators do have a shelf life, your level may be ok ,but if the core is toast it won't cool worth a darn.
318I convertible, Dinan Exhaust and performance chip
Roh BMW racing 17's, Dinan Stage 2 suspension system. Pinoeer CD player, pioneer speakers front and rear. Pioneer 400 watt 6 channel amp. and MTX 8" subwoofer.