2000 Honda Accord Vibration when Braking

I bought this used 2000 HONDA ACCORD LX a few months ago with approximately 76,000 miles on it, and it worked perfectly--until a new mechanic replaced the TIMING BELT at approx. 100,000 miles (per the dealer's routine maintenance recommendations). Now I notice when I BRAKE FAIRLY QUICKLY or as I SLOW ALMOST TO A STOP, there is some VIBRATION that wasn't there before. It seem like the engine is going to die at that point. I have gone back a couple of times, and the mechanic simply adjusts the idle. It seems to help, but doesn't totally solve the problem.
Then a couple of days ago I went back to him for the same problem and also had him replace two sway bar connections (which had gotten rusted and broke--possibly after exposure to the Katrina flood). On the way home on a very hot day after about 20 minutes in freeway traffic which was only crawling along, I noticed my CHECK ENGINE light was displayed. My anti-freeze was not below the MINIMUM mark.
The mechanic's tester showed a P1831 ERROR (as best he recalled when I asked him later). He told me he had called somewhere to see what that meant. However, he had been told there was no P1831 ERROR CODE, so he didn't know what my problem was.
From what little internet research I have done, I suspect my VIBRATION PROBLEM must be caused by an IMPROPERLY ADJUSTED TIMING BELT. He replaced the TIMING BELT a couple of months ago, and I don't want him dragging his feet about fixing the problem until it ruins my engine. What do I say to him to get him to properly adjust the timing belt (which I understand would be once again taking apart the engine)? I think he is just being lazy and is simply waiting for me to take the car to someone else to have the job redone. Also, is the ERROR CODE indicative of some other problem?
Thank you for your help.