I have a similar thing going on at the mo! When driving along and the oil pressure is up, no oil light. When stopped, and at idle, oil light flashes. Had me puzzled till I was under the car the other day, and noticed I have a minor oil leak, which I cant be a**ed to fix yet (its too cold!) I'd say do the obvious firstly and check your oil level, and then maybe get down n dirty, and have a look around the sump and oil pump for signs of leakage. Also make sure your oil filler cap is in good condition and on properly. I could be barking up the wrong tree, but its a starting point for ya!