Cooling Problem

[Mark Purtell <purtell@students.uiuc.edu>]

I just bought my first MGB-its a 1977, and I believe that it is 100%
stock.  The car overheats after driving it for 20-30 minutes, and then it
will stay in overheat for a 1-3 minutes and then return to normal
operating temperature, only to have the whole process occur again at a
later time.  I have replaced the thermostat (195 degrees), and the 
temperature sensor.  I have noticed that the lower radiator hose doesn't
build pressure even when the car is warmed up/overheating.

Any suggestions?

Mark