Don't know if this will help or not. I used to drive a '75 Duster (Slant Six, 225 c.i. with a 1 bbl Holley) with the same problem. Everything was fine until it heated up, then I'd lose acceleration and finally the engine would die. After much investigation, I discovered a cracked exhaust manifold was letting exhaust instead of fresh air into the combustion mix, and effectively flooding my engine with a too rich mixture. When the metal cooled, the crack was relatively sealed, but when the metal heated up from running, the crack (well, the metal around it) would expand, opening the hole that was causing the problem. It took me the better part of 4 months, but I finally tracked down a new manifold in WV - too bad that car got totaled like....7 months later (bad driver + illegal turn + rain-slicked pavement = totaled car). Regardless, if your co-worker's XJ is carbureted, it might hold some bearing. Fortunately, there are plenty of manifolds out there for XJ owners. Best of luck.
