Sounds like either you have exhaust blockage off the #1 bank or the converter for that bank is no longer up to *****. Or both, with the blockage in the converter. Likely replacement time. The warranty on the emissions gear is very long (at least 5 years), and you might still be in it, so I would get down to the dealership promptly to see if they will put in a new one (or two, if you can talk them into one failing means the other is about to) for free.
pax
smn
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'98, Black
Mod.Porter intake, K&N panel, SARC cutout, instrumentation cluster, 23mm rear swaybar, 245x50x16 Bridgestone Potenzas