Toyota Repair: 93 Previa 2wd Automatic Transmission, propeller shaft, forward gear

Question
This week my '93 two wheel drive automatic (196k miles) has developed an intermittent (most of the time) problem when going forward from a stop. After releasing the brake with the trans in a forward gear, the van does not creep forward as usual (the idle speed 2-5 mph push is not there, only a very slight push). Upon moderate application of the accelerator pedal the transmission 'engages' with a BANG and the van jumps forward. I feel a large jolt like I dropped the clutch in a manual shift vehicle. So now I apply the accelerator VERY gingerly so that the 'engagement' occurs almost imperceptably. Once underway the trans operates normally. OD works. Reverse is fine. I can tell when this is going to happen because when coming to a stop I can feel the trans 'disengage' just as speed drops to 3 mph or so.

I flushed the trans fluid again (second time in the last year). No improvement. Trans fluid level is correct. I pulled the propeller shaft out. The u-joints are fine.

Could this be the service bulletin issue involving the check balls? What is my prognosis?

Thanks in advance,
Mike

Answer
There is no service bulletin concerning this problem for the Previa it only applied to the first generation of vans, 1985-1988, check to see if the throttle pressure cable is adjusted correctly, it's located on the throttle body linkage, there is a black rubber boot on it, remove the boot and adjuswt the throttle pressure cable, there is a small metal crimped piece on the cable, it should be about 1mm from the end of the cable housing, if adjusting this doesn't help there may be a valve body problem.