There's no reason to suspect a tire cannot hold 85psi if it's rated for that, but it's possible an old tire is no longer in good shape. As almost all blowouts are related to either poor seating or damage/defect in tire those possibilties should be explored. The guidance FB posted above should help. Just mount a new tube, inflate to about 40 psi, then about 10 lbs at a time, making sure that the tire is well seated with no tube underneath. If the tire's raised rubber bead line next to the rim starts to pull away from the rim in some areas, deflate and recheck for tube or rim strip under the bead in that area, as well as making sure the tire casing is in good shape in that area. If it keeps occuring you may have a bad tire.