truck turns over but sometimes wont start

Try changing your fuel filter, and run some fuel conditioner through as well.

Do you lose spark, and fuel? or just fuel? When you remove your fuel relay can you jumper your fuel pump there ? Then verify spark and have it start? or spark is lost based on temperature? or what. What engine? what year? very important to know before any vehicle diagnosis.

Fuel conditioner is a waste of money, just buy better gas. If your fuel was contaminated or diluted you best get it outta there.

When you say engine turns over you mean the starter cranks the engine but your concern is it does not fire? Or the engine starts and dies immediately? This could be loss of injector signal. You could be describing multiple problems the way that was worded.