1999 Jimmy won't start.

My Truck is a 4.3 Liter Vortec 6. It's starting to get colder and for the past couple of weeks, my truck, has had trouble starting. It used to start almost immediately before the winter started, but began to crank longer before finally firing. The other day, it took me almsot a minute to finally get the engine to fire. It was cold and windy that day and there was also snow. After I parked it at work and let it sit for about 3 hours, the truck wouldn't start period. It would crank until the battery would drain but would not fire. In addition, the fuel gauge on the dashboard does not work; it will either show full or empty and shoot inbetween the 2 periodically. I've also noticed a hesitation with my truck, meaning sometimes (usually when it rains) the truck would only go 10mph and slowly climb while I have the gas pedal to the floor. My mechanic told me he can turn the truck on and off my using the computer that plugs ionto the trucks computer but can not turn it on through the truck itself. HE beleives it to be the spider assembly butI have been told by other people that it may be a ground wire.

What do you guys think it is? How would I go about fixing the problem myself? How much would it cost?