I have a 4cylinder but when mine was doing the 3/4 up on the temp guage befoe going to the half way point on the guage. It ended up being my fan clutch. I even changed the thermostate and it helped for a day but then it started again. When i changed my fan clutch it fixed it and now goes the a little more than quarter way and stays there for the whole time.

88 Trooper. 1.5 inch lift. Rancho's, manual hubs, header, muffler, High flow cat, 2.5 all the way back, Aussie Locker, CB and 31 10.50's