1995 XJ power steering 'boiling' over?

PS fluid won't freeze unless it is contaminated by water. But O-rings and plastic bits get wibbly in cold weather. Hoses can crack with age and 95 is old enough that the return line might be in bad shape. If you replace this line, don't pull it off of the reservoir barb, you can break the reservoir. Instead cut the hose carefully.

Of course you have to empty the reservoir before removing the hose. :) I just cut the hose and stick one end int a jar.

Air or low pressure in the system. see bleeding below

Proper bleeding first. Engine on, wheels full right plus a little press (not much and not for more than a couple of seconds). Full left plus a little press, (just a few seconds). Then check again. Takes a bit for all the air to get out. Might have to repeat this several times.

If that fails then:

Best case is it's a leaking O ring between the pump and the reservoir. Get a leak there and it will suck air into the system.

Worst case is the bypass valve in the pump has failed and you need a new/reman pump. When you change it out make sure to replace the O rings on the reservoir or you will get leaks.