1996 Pontiac Transport SE cranks but wont start

1996 Pontiac Transport SE mileage: 113,000. One day we went to start the van and it just kept on turning over with no sound of it even trying to start. The plugs had a spark, so we suspected the fuel pump. Took apart the fuel line at the filter, turned the key so the pump would come on and sure enough, fuel shot out of the line. Puzzled we put the fuel line back together. Turned the key to start and pumped the gas pedal hard and it started. The problem now is that it now has a tendency to want to cut out especially just after it slips into overdrive, or around 50 mph. Also if you want to pass a car or take off quickly. Changed the plugs and wires. My friend showed me the paperclip trick for the diagnostic code, it revealed problem # 21. Believe this is throttle-positioning sensor. However we disconnected the battery to reset and there is no longer any problem code. Just the vans code of #12 over and over again. Could it be a weak fuel pump? We have also replaced the fuel filter and yes, it is in the right way. My next option is getting a new coil pack and putting it in each of the three places to see if one is bad. I have however unhooked the sparkplug wire at each of the six locations coming off from the coil packs and started the vehicle and had a strong "ground out" so I really don't want to spend the money on it. I am just about out of answers. If you could narrow it down for me, I would appreciate it.