i really don't think it's the pump. is it making any noise? is there any play in the pulley? these are typical signs that the pump is going out (also, leaks but you said there were none). i find it hard to believe that if the pump were going out would be getting symptoms you are describing. maybe someone will chime in with some additional advice, but i'm still putting my money on a loose or glazed belt. they need to be really tight in an XJ.
1999 XJ For Sale - Lifted, Leather - PM for Details