95 idling issue

I have a 1995 mazda 626 AT. Recently I have changed the timing belt when the mileage was at 83K. I have then noticed that when the engine idles, the Car shakes or vibrates. Some times it gets worse when the A/C is on. Some times even when the A/C is on, there won't be that much vibrations as it is when the A/C is off. I also feel the car a little jerky when I am backing it in the Reverse gear. The bottom line is, the car vibrates or shakes when I stop the car at a red light or when it is idling. I have tried with fuel injectors but of no use. Other than this issue, The car runs great. What is causing this vibrations? When I took it to the mechanic (either firestone or Mazda dealer) they say it will take $100 to check the car completely. I don't know what to do. Please help me. vasu