It sounds like it has not had any ECU updates, they ran so bad that there was 4 different ECU's out, if you complained when it was new they would throw in a new one and hope it went away. You can take it to a tuner and they will use the Yoshimira box and write directly on the eprom in the ecu and change the map. I had my 98 tls done about 2 months ago, it will now idle through a parking lot without kicking or bucking and all of the surging is gone. Mine was bad between 3000 and 4000 rpm. do you have different pipes and K&N filter in it because after I put that on mine is when the surging got really bad?