1999 Silverado 4x4 sputtering

I've recently been having 'sputtering' problems with my 1999 323ci Vortec while accelerating from a stop. The 'Service Engine Soon' light came on the other day, but after I put a higher octane fuel in it, the light turned off. I still encounter the 'sputtering' problem, and am taking it in to see if a mechannic can turn me on to what the problem might be. I've already changed the battery, air filter, fuel filter, spark plugs, PCV valve and it was about a quart low on tranny fluid. It also has a coolant leak in the front of the motor, but I think that it might be the water pump going bad, since I can see coolant on the ground. It's got 104,500 miles on it and this is the first problems that I've had with it. If anybody out there has had similar problems, feel free to let me know.
EKG125

Thanks for all the help....it turns out that the fuel pump was going bad. $633.32 later, it runs fine AND my fuel gauge is accurate!