Dropped rpm

When driving, the rpm seem to be correct. However, when I stop, e.g.,waiting for green light RPM drops once in a while to zero (motor is still running). Sometimes, it returns after a minute or so to normal (600), other times it stay at zero. Once I move the car the gage show normal rpm. However, on two occasions when I started to accelerate the engine died. I can restart the engine again and continue. This is unnerving since I never know when the engine will die. What is the problem and the fix for this? Thanks.

Sounds like you have an intermitent wire disconect. Hard to find but try moving the wires around with the engine running. If it dies, look there.

I checked the OBD codes but nothing was there but 55. Another observation. When the AC is on the RPM stays stable like a rock never dropping to zero at idle or in neutral.

Check fuel pressure. You might even have to tape the fuel guage to the windschield and drive until it dies.