i have a 97 f-250 with 195000 miles on it this proble has been a problem prolly for the last 30k miles, the truck runs great all over town and on the freeway but if i'm starting up a hill it flubblers and bubbers especially when i'm pulling a trailer with a good load on it, if i ease out of the gas and ease back into it slow it will usually smooth out and run good again, any ideas on what could be causing this problem? it does it whether i've got the ebpv valve sensor plugged in or not, also it seems to do it worse when it comes closer to an oil change i've read about testin the hpop pressure where do you test it from?