ad 150 wierd fuel pressure

ok i bought the truck and it had 143 thousand on it it now has 155 on it. the vp and lp has been replaced before i got it the lp is a ad150 and the vp is a bosh. i had autometer nv gauges installed right before christmass last year . the truck has allways burried the gauge at 15psi even at wot. about a week ago i started the truck and noticed the fp gauges was jumping from 13 to 14 psi just sitting there. as i drove the truck it would drop up and down and at wot in 4th gear it was reading allmost 11psi. i just replaced both ad filters and also the factory filter on the block and it is still doing the same. i drove the truck today for about 20 miles and the fp gauge was burried like it used to be. then this afternoon i took off and iit was back down again. is ad going south on me. i have check for leaks and cant find any truck is all stock except for a straight pipe. it hasnt lost any power and doesnt smoke any diffrent. any body have ideas thanks a lot