I have a 2000 nissan sentra 1.8 GXE with 97,000 miles on it. And since I bought it Im having problems with it. The most important problem right now is that when I press the gas pedal, the car takes forever to run. And the RPM goes up to 5 and the car still don't run and suddenly it runs normal and it continues to do that for the rest of my trip. When I stop in a light, the car starts shaking like crazy. Then the green lights comes, I press the gas pedal, the car goes up to 5000 RPM and only 10 miles an hour for about 200 ft and suddenly it runs normal.I took it to the mechanic and they changed all the fuel injections, air filter, oxygen sensors, fuel pump, sparks plugs, the catalyic converter (for an used one), they cleaned the radiator, fix the intake gaskets, changes the mass air sensor(MAF). They've done a lot to it and I just don't want to spend more money. Any help.I'll will apreciate it too much.I took it to sevelars mechanics and to the nissan dealer and nobody knows what does it have. CAN YOU HELP ME PLEASE?