engine rough at certain speeds

[vang]

Hey All
I've got a puzzler that you guys will have fun figuring. I have a '91 EB. It runs great in city, and at high speeds at 55+ mph. However, on the highway at a normal 45-55 it is running really rough. It just started doing this. It feels like there is water in the gas line or something.

My thoughts are either

-driveshaft is bent a little
-tranny gears are starting to go
-bad gas
-bad spark

Before I do any or all the above, I'd like to get your advice(s).

 

[mdrut]

First, get a $30 dollar code reader...Will have you time and money...You can pick on up at any Wal-Mart or parts store..Good luck

 

[AgExplorer]

If the fuel filter hasn't been replaced for some time, you may want to try that. I would also take a look at the spark plugs and wires.

 

[vang]

I just had a new fuel pump put in and cleaned the fuel injectors. Is the fuel filter outside the gas tank? Is that hard and expensive to change?