Hi folks,

I have a 92 Trooper LS with the 3.2 DOHC engine.  It has been a
wonderfully reliable and comfortable vehicle for me these past 160k
miles.

About 2 months ago, the check engine light started coming on
sporadically, mainly while driving at highway speed in cold weather.
It would usually go off once the vehicle slowed down to about 30 mph
from 50 mph.  This has continued and seems to be even more random
these days - i.e. it will come on for about 2 minutes while driving 30
mph and then go off.  I've had it in the shop 2x and the computer
cannot find any codes to accompany this.  Mechanic tells me that I
need to bring it in while the light is still on, otherwise they cannot
trace it.  But the light does not stay on for me.

Simultaneously, the speedometer has been acting really wacky.  I'll be
driving 70 mph and suddenly I'll look down and it is sitting at zero.
Then it will jump up and dip up and down crazily before finally
settling back to normal.  This sometimes happens in unison with the
check engine light, but I also see the speedometer glitches more often
than the check engine light.

I'm not noticing any other problems, so I've been just living with it.
Has anyone seen these symptoms and know what the problem is?  Do I
need to get it fixed or is it ok to just let it be, considering the
age and high mileage of the Trooper?

Thanks,
Eric