With that kind of milage, the converter beginning to go would not be unexpected, but normally you can feel the result pretty easily. Particularly on the highway if you have a hilly section. Maybe the computer is getting a false signal from the overdrive system?
pax, smn
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'98, Black
Mod.Porter intake, K&N panel, SARC cutout, instrumentation cluster, 23mm rear swaybar, 245x50x16 Bridgestone Potenzas