Check the banjo bolts http://www.nmia.com/~dgnrg/banjo.htm
Or the check valve in the oil container is gone = no engine oil in the oil container and it takes time to pump the oil from the oil pump to the oil container/pressure valve. Take the cover off after it sits overnight & if it is still full, it's not a checkvalve issue.

this is the new oil filter housing http://www.exx.se/objects/bimmerforu...ilterhouse.jpg

DIY repair http://www.bmwe34.net/E34main/Trouble/OilCanister.htm