Q: Stalling at stop lights

I was at a red light yesterday and my truck began to stall, but it didn’t shut off all the way. The rpms were jumping back and forth on the dashboard and went from 600 down to 100. The Check Gauges light came on and the engine started to rev on its own. Then the rpms went back up. So I stepped on the gas and started driving. It was OK after that and OK in park. At a stop sign or stop light it felt like it was going to quit on me. It didn't happen at every sign or light. I started to think that it might be the oil pressure. It was at 40 psi when starting cold then it goes down to around 10 – 20 when idling and stays at around 20 when driving up to 2000 – 2500 rpms. I haven't driven it over that. I changed the oil pressure switch and the truck is still doing the same thing. Do you know how I can fix this? I would really appreciate it.