Honda Repair: engine, muffler shop, honda civic ex

Question
Hi, I have a '98 Honda Civic EX with over 169,000 miles.  Recently, I took my car into a muffler shop where they installed an after-market catalyctic converter on my vehicle.  The original one burned up on me one morning back in August 2005.  Currrently, the check enginge light is on.  I took it to be diagnosed and found that it says catalyctic below threshold.  The mechanic found out that the spark plug wired were loose, and they did not fit snugly onto the valve cover.  The wires were put on by the muffler shop.  Now, my problem is that sometimes while driving down the highway, the engine misfires.  My tachometer drops suddenly, then jumps around low and high sporadically for about a minute.  Then it will drive right now and then.  I suspect the problem comes from the mechanic from the muffler shop.  Could you please tell me what are the causes to this problem, and what is possibly the remedy.
                 
                 Thank you.

Answer
Nathaniel, is the check engine light still on? Is there power when the engie is running o.k.? Bad PCM/ignition ground? If aftermarket wires or any other aftermarket ignition or emmision related parts were installed on the vehicle there could be some issues. I've seen cars with aftermarket spark plugs that couldn't even hit 3000 rpm with the pedal floored, put NGK's in and the thing ran like a dream. If you think this other tech is shady who know's whats been done to the car. I would recommend bringing it to a HONDA dealer for diagnosis. Sorry I couldn't be of more help. RICK