Picked up this truck about a month ago. Ran fine. Had a few intermittent hard starts but always turned over in about 2 or 3 seconds. This morning it just about fired up and then made almost like a "last gasp" sound and now it Cranks OK but won't start. I noticed while driving that the fuel guage needle jumps around a little. SES light is on. Had auto parts store check it 2 codes came up. One for Evap, guy said to replace gas cap which I did. the other code was for catalyst or something. I assume it's exhuast related as the truck is pretty loud. Light stayed off for about a day or two after reste but then came back on. that was about three weeks ago. anyway I was hoping for some thoughts where to start. fuel system, ignition system??

Thanks for any help.

well after I got the kids off to school I began some caveman style troubleshooting. read some stuff in this froum. tried pouring some gas down throttle body and it tried to start. so i ruled out ignition problems. I could not hear fuel pump turning on. crawled under and jiggled some wires, switched relays in glove box, still wont start. Left to do some running around and price a fuel pump. tried again for sh&#s and giggles a few hours later and this time I could hear fuel pump turning on. Turned key and it started right up. The only difference from the morning is that it is about 25 deg. warmer now. But it was by no means freezing this morning. probably around 50 deg. So is my fuel pump on it's way out and just working when it feels like it or should i be looking at something that is temperature sensitive?
BTW anyone know what that box is just under the filler tube (gas cap) mounted to the frame with what looks like 2 fuel lines connected to it?