Toyota Paseo 1993 engine problems

I have a problem with my car few days ago, I'm not shure if it's the rain that did it or not. Every time my motor reach 2000 RPM, no matter if I push the gaz pedal hard or slowly, the engine goes down to 1300 RPM before rising again and continue to do this all the times. First, the garage has changed my distributor then my ignitor, the o2 sensor ant my TPS and it worked for few days, than start to do this again last day after a freezing-rain, so the problen doesn't came from the parts that the have changed. Anybody have an idea?