I have an '89 Probe and the check engine light is always on, even if I reset it. The diag code is for the EGR valve position sensor. I've cleaned out the EGR valve, but it didn't help. What is really odd is the the light does go off sometimes. When I drive to my parent's cabin at 5000 ft (I live at sea level), the next day the light will be off, and it will stay off until I descend to around 3000-3500 ft. I've tested a lot of the EGR parts and they seem OK, so I'm stumped. Anyone here have a clue as to what I should look at next?

Thanks,

Jeff