Silverado wont start?

my cousin has a 1998 silverado with a 5.7 and every once and a while it wont start. It will run fine for a week or two then all of a sudden it wont start. Well the first time it did this I got a code reader and checked it, it said it had a bad TPS so I replaced that. Then a week or two later it wouldn't start again, came out the next day and it fired right up. Well now its not starting again and I have no idea what it could be. It cranks fine, and it gets spark and I can hear the fuel pump running and smell gas after cranking for a while. Every once and a while after you let of of the key it will hick up and sound like its backfiring through the throttle body but thats it. I thought it could be the Crank position sensor or the camshaft position sensor but not sure were they are located at on the engine and the code tester doesn't say anything about them. What could this be?