My 1994 Sierra has about 187,000 miles on it.
When I get above 65-70 mph it starts to miss, especially going up hill and after it has been running for a little while. Not sure if it starving for fuel or flooding or just misfiring .
Also, has developed a slight miss when it idles in gear usually after it has been running for awhile.
Also, started to shift into 2nd gear somewhat abruptly and overdrive quit working most of the time.
Mechanic has replaced most of the ignition system and given the truck a tune-up and last week I dropped $1900 into a remanufactured tranny.
Result: Still runs rough at highway speeds. Still drops hard into 2nd but overdrive now works most of the time.
Any suggestions and could the two problems be related?
Thanks!

Mark

I would check the EGR to see if it is opening too far under a load..
Next step is to perform a compression test..both a static and running test..
If the engine has compression issues, all the parts you throw at it will not fix the problem

I'd check the throttle positioning sensor for the shifting problem. Then I would pull the distributor. There is a shim at the bottom of the distributor that controls advanced timing. That was the problem with mine and took me about 3 months of talking to the right people to arrive at a real resolution.