Can someone please help me troubleshoot before I go in the wrong direction? I've got a 1999 XK8, the top radiator hose is suddenly hot, soft, and certainly about to fail, the bottom hose is always cold. My wife drives the car 1 hour each way to work, the other day was 5 degrees, and the heater stopped working on the way into work. The engine temp stayed in the the middle of the gauge most of the time, but it did shoot up very high a couple times, but then returned to the middle before she was able to get it home. The low coolant light was on-my XK8 has a small leak...somewhere. I've been adding a very small amount of coolant once every 45 days or so(its been doing that since I bought it over 2 years ago, I can still live with that for a while). If I cycle between full hot and full cold, the heater will work for a few minutes, and then go cold again. I replaced the thermostat 2 years ago.
Since the bottom hose is cold, could the radiator be clogged, or is that a sign of a faulty thermostat?

Sounds like low coolant level and trapped air in the system. You're playing with dynamite driving with a known coolant leak.

Wow that upper hose is gonna blow. I would talk to the guys over in the XK8 section but the 1st thing I would do is drain the system , replace the thermostat and replace the upper hose. Have someone do a power flush and refill with antifreeze and bleed the system.

Dang you're right, I thought I posted this in the XK8 section. I'll swap it over.

Probably worth changing the thermostat as a first step, since it's cheap & easy. And see if you can get the leak fixed, too.

The pump impeller on the 4.0 cars is made of plastic, and at this age, it tends to disintegrate (mine did). If the pump has't been replaced for a while, that's probably worth looking at if a thermostat change doesn't work. I believe the later metal-impeller pump fits, but you should check that.