Originally Posted by Mike
I've seen a completely clogged cat cause low performance/sluggish, but never cause it to run rough.
How'* your coolant level?
Coolant Level........good call, Mike. I've seen misfire symptoms occur with low coolant quite often.
One other thing to check is the dash temperature guage. If the coolant level is appropriate, but the dash guage indicates a "cold" temperature, the problem could be a bad coolant temperature sensor. With no coolant temp input, the ECM "thinks" the engine is running at -40 degrees causing random misfires to occur.
Had this prob on a TrailBlazer a while back when I replaced the head gasket.......forgot to reattach the temp sensor and didn't figure it out for a day. Reconnected the temp sensor and the misfires went away immediately.