(T) blowing hot & cold?

hello everyone i need help again got another strange fault.(1994 punto 1.1l)
when driving if i turn the heater on i get hot air, but if i stop and leave engine running it starts to blow cold by its self. it dosent seem to be over heating the cooling fan cuts in and out but still cold air. then if i drive of again it starts to heat up. the temp of the air seems to change with the revs of the engine as when driving and holding around 3000-3500 revs its really hot reduce revs it cools a bit , leave it ticking over and its cold.
can anyone help? i was thinking it might be a doddgy water pump? but not sure.

i would say water pump as well

is there any way of testing if it is the water pump without removing it? any idear of cost of new one? thanks

theres is another post by escotia just below has exact same prob on there w reg. when drivng hot air when idle cold air.

i had this on my old cav and it was the pump the blades on the impeller had worn.corroded away with age soo couldnt push enough water unless going like mad.

not sure how you can test with out removing

I recently paid about 20 quid for a pump at a local motor factors. I changed it cos the shaft seal was leaking, but when off could see the impellors were starting to be corroded away.

Perhaps you could get some idea of the circulation by leaving the header tank cap off and watching as the engine warms up, but it might be difficult to tell.

Could be worth first flushing out the heater circuit and bleeding the system to get rid of any cr@p which is restricting water flow and see how it goes.