Lexus shudders while driving. Is catalytic converter the most likely problem?

Took the Lexus in and the repair person said it was just a loose spark plug. He tightened the plug but we test drove it and symptom was still there. He opened it up again and found that a spark plug wire had damage and was jumping spark to ground. He replaced both caps and rotors and put new plug wires on--said that the plugs looked fine. After putting everything back together he said it made a horrible noise. He went back in and He found that the tensioner jumped the track and put a couple of razor-like cuts in the timing belt. $700 isn't bad for the work but I'm not sure that the work done didn't cause the timing belt problem. He said the water pump looked fine so he didn't replace it. The engine was replaced at 179k with a used Japanese engine by a very reputable shop. The Lexus now has 231,000. We picked up the car and had a 6 hour drive that day. The temp started climbing, a symptom we had before, but stayed hotter for a while even when driving faster. It settled down on the freeway though but now we didn't have the heater fan work properly. By the time we made it back home the temp panel controls were hot to the touch and still not warm enough in the car. 10 minutes before home the outside temperature gauge changed from the correct 41 degrees to minus 22 degrees. Frustrated and not prepared to spend more right now.