Oil pressure light then truck stalls out

I start the truck and let it run its fine but after you shut it off and let it sit then start it the rpm drops, you have to put your foot on the gas and the brake when at a traffic light. The oil pressure light comes on then the truck stalls when the ac/heat is on but it does not do it all the time. Can someone please help me need to fix as soon as possible winter almost here thank you.

Tom