Here's my mystery problem:
Occasionally, at any speed or idling, the tacho will jump about or go up to
4000-5000 rpm's, regardless of what the engines actually doing.
At the exact same time this occurs, the engine will run poorly. If idling,
it will increase rpm a little (100-200) and run rough. If at speed, it will
sometimes hesitate. If accelerating, it hesitates and becomes jerky. The
problem is effected by road conditions.

The same day this started, something else occured and I not sure they are
related. At idle, the Oil Warning Light and Buzzer came on. If I rev'd the
motor up a little, it went away. This has happened 4 or 5 times. It's my
understanding that the two don't come on together unless the engine is over
2000 rpm. This is not necessarily happening at the exact same time as the
above problem.

Another bit of info possibly helpful is that it seems to be idling with an
excessive HC level (190-250). At 2600 rpm it is well within spec.
I just replaced the injectors, thinking perhaps a lean mifire was the cause
of the excess HC but I haven't had it retested yet to see if that corrected
it.

Thanks in advance for any suggestions.