'94 AT DOHC Chk Engine and Cutting out

I have recently run into a problem with my Trooper, actually 2 problems and hopefully related. The Chk Engine light will come on after going down a long hill where I live and stay on until I restart the vehicle. Then it stays off until the next day. Recently the Trooper starts running rough and having slight backfires when going up that same hill. I found that the plug wire on the front passenger side (I am not sure which cylinder that is) would arc on the "coil" occasionally. On checking the plug wire I found that the wire was loose and I could not get it tight. Rather than spending the $60 for a new set of wires I used a small aluminum foil shim to ensure that the plug and wire were tight. However the skipping and Chk Eng problem still remain.
I went to Advance but they told me that their scanner only works with '96 and newer vehicles. I am guessing that mine uses ALDL as the connector (2 of them in a plastic housing) have much less than 16 pins.
Any ideas on what this could be? I have used "high test" with octane boost, etc. to make sure the fuel is clean. Also I had a complete tune up probably about 15k miles ago. The oil has been recently changes. She currently shows 175k miles. And the speedo went out about a month ago. I have the new one but have been reluctant to rip out the entire dash to replace the speedo yet. Does anybody know a shortcut on that?

Clogged cat convertor maybe??