Your problems sound identical to my woes.My 86 760T acted in the same manner.Fuel consumsumption went way out of sight.I had checked all of my vaccuum line that I could get to and replaced the a couple of them .The problem seemingly went away for a couple of weeks,then reappeared again.I finally had to take it into the shop.They did all of the checks and informed me that impulse sender in the distributer had bare wires and was shorting out occasionally and needed to be replaced.While the distributer was out they replaced all of the seals.This apparently is not uncommon for these years of 760 s .I am also aware that I have a biodegradable wiring harness that will need to be replaced soon.I hope this helps in your quest for info.