1997 Chevrolet Venture - Rough Starting

My 1997 Chevrolet Venture 3.5L is sometimes difficult to start....turning the key the engine cranks but sometimes will buck, maybe even backfire. Other times it will crank, though it seems it takes a long time to start. Other times you turn the key and it starts right up. It has 94K miles, I don't believe that the plugs or the wires have been replaced. I plan to do that soon.....I understand that it is difficult to get to the rear plugs. Even with the hard-rough start, I do not get a Check Engine Light. I guess I could connect the OBD-II reader and see if there are any stored codes. The fuel filter and air filter are now as well. It seems to idle ok, once it is running. Any other ideas what might be causing the hard-rough start? Thanks in advance, Joe