[Bob Lincoln]

It's called a 'catalytic' converter.

Do this first - check fault codes and report back what you get: http://www.allpar.com/fix/codes/index.html

It could be many things, but it sounds like the mechanic did not do a proper diagnosis. One possibility is the oxygen sensor. It is not used for the first two minutes after a cold start, but then can cause bucking and poor gas mileage (at least 1/3 lower than normal).

 

[Bob Lincoln]

I'll bet that the wiring harness behind the valve cover has cracked insulation and wires are shorting. Check the whole length carefully. This is common.

 

[Bob Lincoln]

Sorry, I replaced my harness with a spare from my old car before I junked it, the old harness is long gone.

 

[Bob Lincoln]

Teflon wire is good to 200C and is best for this. But even polyethylene insulation is good enough, if you use the corrugated plastic shield over it. Don't use PVC insulated-wire, it's only good to 60C (140F) and will soften and melt together.