Hello, I am new to this forum and not sure if this is the place to post this diagnostic problem but here it is: My '88 GMC S-15 has been acting up: [Just started yesterday.] Sputtering at around 35-40 mph. [I think the truck shifts into 3rd gear at this speed.] Mainly when going up hill, and not so much when going down hill or on level roads. The last time I had this problem I found a dislodged vacuum tube and the problem went away once the tube was re-connected. I naturally checked this area again but found all vacuum tubes attached. I have also considered water in the tank and will be treating for that today. I checked the spark wires and found some to be warn and will be replacing Wires/Cap/Rotor today. If anyone has any ideas on anything that I may have over looked I would appreciate hearing any ideas for other items to check. Another symptom is that I can feel the truck begin to shimmy just before the sputtering starts up. This leads me back to thinking it's a vacuum issue but I do not find any loose tubes. Thanks for any suggestions on what to look for.