The bottom end of those motors go forever. I have done valve jobs on many with that amount of miles.
You need to identify which cylinder is misfiring. The reasons fall within the basics or combinations of such. The basics are: compression, fuel, ignition and throw in timing. If you are worried about the integrety of the motor a compression/cyl leakage test should be done. Compression problems are almost non-existant on these motors but you do have a lot of miles.
Common single cylinder misfires at idle occur due to: ignition components (mostly plug end resistors), injector failures (usually due to trash), and vacuum leaks.
[This message has been edited by stevebfl (edited 10-20-2000).]