Sputtering problems at most speeds

I have a 1994 GMC Jimmy Vortec 4.3L TBI it will cough and sputter in most gears and usually when I go to accelerate. I have cleaned the throttle body and the fuel system, changed fuel and air filters, spark plugs are fairly new along with wires, changed cap & rotor a few months ago, and checked to make sure the catalytic converter was not plugged. It only started to do this when I would tow my boat at highway speeds but now it does it all of the time.(especially when it is hot)It idles fine and first gear has no problems. No codes come up for the check engine light. The manual I have for it does not have the same engine option and timing procedure is a little shady. I have a good timing light and I know it should be zero degrees BTD It says to put distributor ignition system in bypass mode and I am not sure of that. Any help would be great.