Starting Problem H E L P!!!!!!!!!! PLEASE

I have a 96 Chev. 1 ton dually with a 454. The truck starts very easily in the morning or when it has not been driven in a while. The gauges are all in the normal range when driving. Once the truck is turned off, it is almost impossible to re-start. If you let it sit for several hours, it starts up easily again. The problem seems to occur when the truck has 1/2 tank of gas or less. Any suggestions would be appreciated. The truck has 98,000 miles on it. Thanks, Mary Please e-mail me your suggestions. Thanks again.