2001 Pontiac Montana acts like misfiring or cutting in and out

Hello, Could someone give me some insight on what is going on with this 2001 3.4 montana. I changed the leaky intake manifold gasket. Got it back running and now a new problem. It starts up just fine, but when i drive it and it is cruising about 1500-2000 rpm it starts hesitating and jerking and when I pull to a stop it dies. It starts right back up and does the same thing over again. Seems to idle ok it is just when you drive and are at a steady speed is when it is noticeable.
I have changed the fuel pump, fuel filter, new plugs, new plug wires, all three coils, fuel pressure regulator, oxygen sensor (before cat and after cat). changed all of the fuel injectors, changed MAF sensor. When I changed the spark plugs they are extemely black no oil, but they look like they are running rich, it gets horrible gas mileage (9.4 gallons of gas to 66 miles). Would anybody like to take a stab at this situation and give me some other insight on what I might try. Thank you in advance.

I have 1 2005 Montana that is behaving the same way and after some research I found a website that described the symptoms as being a problem with the throttle position sensor. We have been to two mechanics, changed some sensors, spark plugs and wires and they still scratch their heads. The check engine light will not come on, no matter how bad it gets and it is progressively getting worse. So, I plan to take it in and have them test the TPS but before I do, I was wondering if you had resolved the problems with yours. It is apparently only about $80 for the part with labor which is only a fraction of what we have already spent. :banghead: