Lbeezy410

I was driving my truck today when i get to a red light and the truck begin to stall like it was going to cut off but it didn't. I look at the dash board and the rpm is jumping back and forth from maybe 600 down to 100 (Check gauges light comes on) engine like revs on it own then bounces back up until i hit the gas and drive then its fine, it also seem to be fine in park to just when in drive and on the brake like at a stop sign or red light it felt like it was going to cut off and this didn't happen every single time. Now as far as the oil pressure which i originally thought was the main problem and still could be is at 40 psi on a cold start...then it goes down to about 10-20 when idle and sits at about 20 when driving up to 2000-2500 rpms haven't taken it pass that. So today me thinking it was the oil pressure i got the oil pressure switch replaced today and its still doing the something after 100 dollars gone (overcharged) 51.00 dollars for the shop to order the part should have picked it up myself for 30 dollars but whatever it didn't work and everything is still where it was and Its still stalling sometimes when i stop with the car in gear at a light ect.... any idea what it could be. Any help will be appreciated

Truck: 1999 chevy c2500 5.7 engine......171,000 miles....I got the oil changed a week ago they put 5w-30....pressure switch changed today