fauilure to start every time

I own a 1990 GMC Sierra 1500 pickup, 5.7 engine-350. I am having a problem getting it to start every day. I have had it is the shop many times for this same issue, they find nothing. I have replaced, air filter, fuel filter, fuel pump, fuel pump relay, ignition module, distributor cap, coil, rotor, pickup coil, plugs. If it isn’t driven it will start every day, but then when you start to drive it, it might not start every time. There is only about 101,200 miles on it. I can use starter fluid on it and it will start but here lately that’s not working any more. But come back in a hour or two and it will start. I love this truck and would like to get it more trust worthy that it is. I thought that it was weather related, but it just seems to have a mind of its own. Do you have any thoughts of what this might be and what it will take to repair it. It might start for weeks and then all of a sudden it want start for the next two or three days with out help.

Thank You,
Chad