2000 GMC Sierra V8 4.8L - I recently had my fuel pump & fuel filter replaced. I just changed my spark plugs and wires. Truck starts up fine, but it idles low now. When I'm driving it now hesitates some when I'm giving it gas, but if I floor it, it doesn't hesitate much. I checked all the wires to verify they were'nt loose. Any idea what to do?

Clogged fuel filter / clogged injectors / dirty air filter.
Fuel pump could be going bad.
Loose ground somewhere.
I have a 2000 GMC Sierra. I have replaced the ignition switch (its more of a harness than a switch).
If you have a check engine light on, this will help diagnose the problem. If you do not have one, then it is most likely a mechanical issue like a bad fuel pump or a loose ground someplace.
Things to note:
How long does it run before it shuts down? If its the same amount of time each episode, then it could be related to temperature.
Do you have any leaks? Check your fluids and make sure there are no leaks. These trucks have many safety features built into them that are dependent on fluid levels.
Any blown fuses?
When is the last time you changed the air filter, the fuel filter or the spark plugs?
Air filter should be changed / cleaned if you have a reusable one every other oil change.
Fuel filter should be changed every 30,000 miles (I may be wrong on the mileage, but if you do not remember when it was changed, I would change it. With the age of the vehicle this is more of a factor - even if the truck has low miles on it.)
Spark plugs are expensive but only need to be changed every 100,000 miles. Given the age of the vehicle, I would guess you are not the original owner and without knowing when they were last changed, it might be a good idea to "start over" with everything being new. These trucks will last forever if you take care of them. My Sierra has 217,000 miles on it and I just towed a full size truck 1500 miles with no issues. GOOD LUCK

run the fault codes
could be a problem crank/cam position sensor or bag ground wire connections

If you took the bed off, you would have had to disconnect the ground wire at the left rear frame rail. This is the ground for the fuel pump...The fuel pump won't run without it.

Check the timing and make sure the battery is good, the battery can sometime have the power to crank the engine but have internal problems that leave it without enough voltage to give you a hot enough spark to start the engine.
You could also have some bad ignition parts which could be shorting out your spark or sending it to the wrong spark plugs. Plus you could also have fouled spark plugs.

ROUGH IDLING CAN BE CAUSE BY MANY THINGS.I AM LOOKING AT LEAKING VACUUM HOSES OR INTAKE MANIFOLD GASKET LEAK.DIRTY FUEL INJECTORS AND WORN ENGINE VALVES WILL CAUSE ROUGH IDLING.ALSO LOW IDLE SPEED CAUSE ENGINE TO RUN ROUGH.

that has a votec engine in it the fuel pressure is droping at knight and it takes 60 psi before the injectors will fire most likely cause fuel pump ,leaking injector ,or a feul pressure regulator

check fuel pump relay outside under hood there is also a pump jumper on driver side single red wire with plug on end where harness comes out of firewall hook up to bypass and see if pump is working also check to see if you plugged pump back in when replaced

its your fuel pressure regulator!!!!!!!
change that!!!!!!

It could be electric fuel pump stall. The pump isn't pumping fuel fast enough. This model may have a inline fuel filter before the fuel pump as well that might need replacing.

Happens all the time with 70's 80's and 90's GMC trucks. Replace the control module in the distributor cap.