Truck Forums - Starting Problems

O.K. I own a 2000 GMC Sierra V8, and I've been having problems starting my truck. It will start great first thing in the morning, but after that, it doesn't want to start right. I have replaced the spark plugs and wires, but there was no change, I've also taken it to get a Diagnostic, but nothing showed. It has been sitting for about 5 years before I got it, It was hardley ever driven untill I got it. I also noticed that If I push on the pump while I'm trying to start it, it helps. I don't know if that'll hurt anything, but if so, please tell me! I also noticed that when I am stopped at a light, the truck will shake and try to shut off, and it will do that in park and when I put it in reverse. O.K. I think thats it, now that I've written a whole book! Thanks for any and all Help!!