Please help...very puzzled. I have a 1996 c150 and have never had a problem until now. The last few times i have gone out, i have encountered this:<br /><br />When i start cold, the engine runs fine....i have no problems on idle. when i run for about 20 minutes (at 4000 rpm) and then drop back down to idle, the engine starts to sputter and act like its going to turn off! i checked for water in the fuel, but its fine.<br /><br />Furthermore, when i try to increase rpm, the engine is very sluggish until it reaches about 2500-3000 rpms.<br />To make matters worse, it only happens when in the water(load), never during a flush.<br /><br />????? 