Toyota Repair: Sienna 98 Transmission, accelerator cable, automatic transmissions

Question
My Sienna 98 transmission with 106k miles is shifting hard in 1st and 2nd gear.  I was told it was my motor mounts which I had replaced but the problem is still there.  The transmission was flushed but problem still exits.  Aside from rebuilding do you have any faith in products like Slick 50 for automatic transmissions.

If it helps when we had a few cold days (high 30s low 40s...live in Orlando,FL) the shifting was more smooth until.

Any thoughts are appreciated.

Answer
There were some problems with these early model Sienna transmissions, I assume there is no check engine light on if this is the case then it's a mechanical problem not an electronic control problem, the first thing to check is the throttle pressure cable adjustment on the throttle body, the throttle pressure which controls line pressure and can cause shift shock if not adjusted correctly or if the cable is stuck for some reason. The adjustment can be checked where the cable connects to the throttle body, the accelerator cable also connects there so don't get the two mixed up. The throttle pressure cable comes up from the transmission and connects to the lever next to the throttle cable, it has a small rubber boot on it, peel the boot back, there is a small metal piece crimped on to the cable, this metal piece should be 1mm away from the cable housing( the threaded part)if not loosen the 14mm adjustment nuts until it's at the specified distance. Make sure the cable doesn't bind up when moving the throttle, it should return all the way back in when releasing the throttle. This is the most common cause for hard shifting, if that doesn't help then there could be an internal malfunction with the transmission and replacement would be necessary, personally I don't have much faith in additives, if any help at all it will be temporary.