1996 Jeep Cherokee Trouble Starting

I have a '96 Cherokee Country with a 4.0L and about 170K miles. I'm having an intermittent problem. Sometimes the truck starts normally, but occasionally it just cranks and won't start. When this happens I found that if I depress the gas pedal slightly while cranking it, the truck will start right away and then run normally. The problem seems to happen mostly when the truck has been sitting for a short time (less than 30 minutes), for example when I am out running errands. Any ideas? I already have a new onboard computer, fuel pump, fuel filter, and neutral safety switch, so those things can be ruled out. Thanks in advance for your help.