So, ive had this problem a few times before, it'll start initially, rev up for a second and die. the first time, i changed the fuel filter and it ran fine, the second time i ran out of fuel, filled it about half way, and it ran fine up until like 30 minutes ago, when i had the rear end up on jack stands, it stopped running. it seems like whenever its on an incline, it doesnt want to run. does this mean i could have a bunch of crap in my tank? any suggestions otherwise?

p.s. its an 88 xj with a 4.0 in it