I don't know much about E34's but if it is a traditional style rad cap then sounds like you have the wrong one.
Cooling systems work under a lowish pressure and therefore the rad cap must be able to withstand that pressure. If the rad cap can't take the pressure it will blow and hey presto the ground beneath your 6 pot gets a showering of coolant.
Years ago when I drove a Mini the rad cap had the pressure stamped on it (15psi) which meant it would blow and let out the coolant when the pressure exceeded 15 psi.
BMW should be able to tell you what pressure rating your rad cap needs to be and also be able to supply you with a correct rad cap.
Sounds like she maybe got a bit hot on your 2hr drive and that was enough to increase the cooling system pressure enough to blow the cap. Something must be causing the pressure to rise to get too high without it over heating. Try flushing out the system as it may be blocked. Also make sure the system isn't overfilled otherwise you will get coolant escaping from the rad cap as it expands and flows up the rad and out thru the cap.
Don't run without the stat for too long otherwise you will suffer from over cooling.
Hope this helps
Andrew