Throttle position sensor may be faulty. They do not always show fault codes.
There is a vehicle speed sensor too that may also be at fault. but the TPS is easier to get at initially.
SOURCE: 1992 Lexus LS400- slowing down on interstate for
Had the same problem with mine...replaced the fuel pump...worked like a charm...
SOURCE: 1993 Lexus Ls400 loss of power at 2500-3000rpm
Check your MAF and MAP sensor. The first 80% of your throttle signal from the TPS works with the O2 sensor to try to keep the air fuel ratio to 14.7 to 1 with the ECM controlling the fuel injector spray time and timing the ignition. After the 80% to WOT the ECM puts the O2 sensor in to an open loop and takes the Reading form the MAF and MAP sensor to run the engine richer so there will not be any Detonation to the engine and burning holes in the valves for running too lean. That is why it always feel like you get a little more when you ease off on the gas peddle from WOT. Recap check the TPS, MAF,and MAP sensor and wires to those sensors, if you can get the car scanned that would be great and you may find out which sensor may have gone bad, the other thing to is the fuel pressure regulator may be going bad and at WOT the pressure may be to high causing it to run to rich causing the loss of power. Good luck and hope this helps.
SOURCE: 1993 Lexus LS400 acceleration stalls or surges
I HAD THE SAME PROBLEM WITH MY CAR. 93 LS 400. THAT WOULD BE YOUR FUEL FILTER OR THE FUEL PUMP GOING BAD. TRY CHANGING THE FILTER FIRST. THE DEALER TRIED TO CHARGE ME $980 TO CHANGE FUEL PUMP.
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