Every time I come to a slow down or a sudden stop the car starts jerking back and forth and stalls.

First, the mileage is in KM's. Now, I have a HUGE dilemma. My mechanic and myself have been working on this for the last 6 months and still can't figure this out. I have changed the O2 sensor. I have changed the thermostat. I have changed the Idle speed control. I have checked for any loose timing. It's 100%. I have done the fuel pump and filter. (The filter twice). Changed the wires and plugs. Also replaced the manifold exhaust gasket. I still have the same problem. I sit for a minute, shift to drive and it stalls again. I have to let the car sit for at least 10 minutes before I can even get it to go into drive. Once I get it in to drive it shakes like it wants to stall again but when I punch the gas it takes off rough. While I'm driving it shakes back and forth. (That's why I checked the timing, I thought it was skipping)
Oh, and the EGR valve is good. I'm lost. Is it a possibility that the fuel pump is defective. Also, I noticed that the previous owner had redone the power pack. According to the manual they are in no way near the same order. Maybe it's the power pack. Please help. :(