2 quick questions for you. 1) have you checked/replaced your radiator cap ? I didn't see that on your list. and 2) is the cooling fan functioning ? you said you were suspect of the sending units but never mentioned if the fan was responding to the temperature level.adding/checking the temperature with a second guage may be a good idea.
My 90 started heating up on me one day and after some painful troubleshooting, I found a corroded wire on the relay for the cooling fan over by the battery.
