put a vacumn gauge inline and watch it what it does...Ill bet like people have said your pickup is plugged up in the tank, when you drive it collects more and more and under a heavy load it clogs up tight...ie no pressure...Ive got a customer with a diesel van that this happens to often...ive cleaned out the strainer more times than I care to remember. Its a ford van but symptoms are the same...thats where id start!