Get any fault codes stored on the ECU, they may give you an indication of the problem.
1) Clear the memory by pulling the 20amp efi fuse in the engine bay junction box (check under lid for position) for at least 10 seconds.
2) The 92 ECU has a super sensitive mode that is used for diagnosis, you simplay short together terminals TE2 and E1 in the TDCL diagnosis terminal in the footwell before starting up, go for a drive and it will store the codes from its analysis.
3) with ignition on you connect TE1 to E1 and any error codes will flash out on the check engine light in the combination meter. Count them, ie 2 flashes, pause, 5 flashes is error code 25. Sequence runs from small code numbers up, then repeats.
4) report back here with the codes! its an easy way to keep your lexus dealer honest too!
ok.. I will try ...before I go to the dealer...but looks fairly technical