Mine usually runs rock solid at 200 unless I am heading up the mountains where it will hit 210 but drops down instantly to 200 as soon as I hit a downgrade ( that is with a 190 degree thermostat.) I bought a replacement rad at Advanced and it is 100 percent copper. I don't know if my 90 YJ is different then your 91 though?
1990 Jeep YJ 4.2
Howell EFI, BDS 3.5" lift, M.O.R.E 5/8" Boomerang shackles, 1" Daystar BL, 1" JKS MML, TFI, Crager soft 8s w 35" Dunlop Mud Rovers, Ford 8.8, 4.88s, Dana 30 E-Locker, Terra Flex SYE, Tom Woods CV shaft, Warn M8000.