Newcastle

I have a 2002 Trial Blazer with just over 200,00 miles. During stopped for traffic, I have noticed that when I stop for a light. The oil pressure gauge drops almost to zero, the the Stop Engine message is displayed. If I punt it neutral and rev the the motor. The gauge pumps up to normal. The stop engine light is off. The next light the oil pressure is normal and will remain normal pressures while on the freeway. I will make 2 -3 stop lights, still normal. the next light I will be at a stop and the pressure, stop engine goes back on.I rev it up and it's fine.

What could be the cause or causes by? I'm the Original owner and I would like to give it my daughter to use for school

I change the oil every 4000 miles

I'm starting to feel a slight slip in the trans mission ( very slight)

My question is what will cause the oil gauage go up and down the way ir's doing.

Thank you,
Robby