Could be air pockets, Renix closed systems are notorious for it.
I would be suspicious of the fan clutch. Do you have an efan?
Anyway, warm the engine up fully, shut it off, and then try to spin the fan. If it spins easily, the clutch is shot. I always recommend upgrading to the ZJ HD fan clutch, NAPA part number 272310, $44, direct fit replacement.
You can check for air trapped in the system by parking nose-down, then remove the temperature gauge sender at the rear of the head to see if any air comes out, or add coolant to the pressure bottle until it flows from the sender's hole. Things will be hot so be darn careful, or do it with the engine cold. Next time you change a thermostat drill a couple holes in the flange part, 1/8" or so, far enough in from the outside edge so the housing and head mounting surfaces won't block them, then position the thermostat with a hole at 6 and 12 o'clock.