98F150,no oil pressure at idle in gear.

New engine, 4.6Lv-8, about 3000 miles on it. I noticed that after engine got warmed up it would idle at about 600rpms and to my alarm, the oil gauge arrow would go down to zero and my oil light would come on. Give it gas or shift to neutral(auto trans.)and oil pressure would go up. No alarms or other noises. I'm thinking that maybe oil pump drive is broke but I'm not familar with this engine. Truck is 4wd.