What causes that drop and what needs to be done to fix it?

I have a 2002 Nissan Sentra GXE 1.8l. I noticed that the RPM in my car drops from 700 to 300/350 and it shakes a little while stopped at a stop sign. Fuel injector cleaner was used and it did not help the situation. After a few months, my car stopped when I was driving and after 10 minutes I was able to start it back up. After driving for another 1-2 miles, it stopped again. SES light appeared. My mechanic replaced the fuel pump, crankshaft sensor, and cam sensor. There was no communication between ECM and checking computer. The dealer replaced the ECM and programmed it. My car's RPM began to go down very low whenever I was driving and had to stop. It was towed and taken for a check up and two codes came up: one for EGR, and another for ignition coils. Only the EGR was replaced. There are no codes now. After starting up, the engine stays at 700 RPM, but after shifting into driving gear it goes up to 750 RPM. After several minutes of driving, the RPM drops to 300-350. While driving everything seems fine, but after another stop sign the RPM drops once again to 300-350 RPM.
P.S. I am sorry for yesterday's switching question with details.