One thing I had to replace when I had rough idling/sputtering engine issues was I replaced the TPS or Throttle Position Sensor. I actually picked one up at Oreilly's for a 1991 K1500 Chevy Blazer and it fixed all my issues. I have seen a lot of other threads that say the same thing. The TPS on the Assault Engines go bad around the same times on the same year models. (I figure it's because most sit through the winter season. Here was some recent info I posted about the TPS:

If you take the TPS off it is spring loaded. When I turned my old one with a screw driver it would stick on the idle side of the spring action. replaced mine and she runs good as new. Nice and easy idle at 800 RPM's. Just make sure that when you put the new one on turn the TPS to the left (or towards the front of the boat counterclockwise) to get it onto the small keyed shaft and then turn the TPS to the right to line up the bolt holes (This will load the spring tension). My throttle body had a small plate under it and I simply bent it down to get the TPS on and then bent it back up to flat gently. Very quick, easy and cheap fix. I got my part from Oreilly's and used one off a 1991 K1500 Chevy Blazer. I have the 5.7 Indmar Assault. Hope this helps you out!