I have about 120,000 miles on the car. None of the check enginge lights come on and the transmission was serviced at about 90,000 miles.
Bucking during down shift. on 1998 Lexus GS300
I have a 2001 GS300 with 117830 miles on it. My transmission had been missing shifts from 1st to 2nd and sometimes 3rd to 4th. I had dropped the pan, cleaned it, put a new strainer, and put new fluid. It kinda helped but not really. Then I noticed when the car was cold, it would hesitate for ten to fifteen seconds when I would floor the accelerator which meant my plug wires were probably in need of replacement. They had never been changed the life of the car so I bought a set NGK Iridium plugs and NGK wires and now, there is no problem shifting. You might wanna consider getting some Universal ATF Fluid from AMSOIL, it will help your tranny shift smoother as well as decrease internal wear. I hope this helps.
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Check the transmission oil first, bucking on acceleration could be ignition related but bucking on deceleration is unusual, the throttle is closed, you have no demands on the ignition or fuel system. Any "Check Engine Light" or pending codes stored? I have seen some transmission problems but not what you describe. How many miles on your car, and when was the transmission pan last removed and filter changed?