The symptoms sound like they could be due to a bad oxygen sensor. In any event, at 100,000 miles, it would be a good idea to replace it anyway. My car has 81,000 miles, and I just checked the sensor according to the voltage tests in the Bentley manual.
Pulling an injector plug and pulling the vacuum hose off the fuel pressure regular tested OK for high and low voltage response, but with everything hooked up normally, the voltage pulses took two seconds to go up and two seconds to go down (oscillating between .3 and .7 volts). It ought to be more like a pulse every half second. A bad oxygen sensor that produces a too rich mixture can ruin tha catalytic converter ($!).