WheelinJR said:
I was under the impression that I would encounter different symptoms with that problem.
Not necessarily. A fan clutch is, natch, a "viscous" clutch (rather than a direct mechanical coupling.) Therefore, it allows the fan to spin at a percentage of the crankshaft speed (which is divided slightly by the size difference between the crank pully and idler pully, but let's leave that alone for now...)
When you sit at a stop and run up your idle in Neutral, you're letting the fan spin at the same percentage of a faster basic speed. Say it's running at 70% of driven speed - 30% of 1500rpm is still twice as fast as 30% of 750rpm, so you're going to have a faster fan.
I've also noted that fan clutches seem to be good for about four years - five tops! - on an XJ. So, if you haven't replaced it yet, you probably should (and take a Sharpie pen to write the date on the new one when you put it in - it will simplify troubleshooting later. A good idea for most "maintenance" parts...)
There should be no difficulty with your 1991 using a 180* thermostat - that just means it opens to circulate coolant earlier. Slightly longer warmup times may result, but you shouldn't have any trouble other than that.
You shouldn't have caught an air pocked with your 1991 either - and it's not presenting as something like that anyhow.
Change the fan clutch - that's the first place I'd look. You've already done the fan override - even though the mech fan seems to move more air - but the symptoms indicate a fan clutch to me (I'm also going to assume that, since you had your cooling system torn to bits anyhow, you've already flushed the crud out of the system as well?)