Bizarre cooling problem

Gents I have a cooling problem on my 4.6 v8 I have removed the gas system and replaced the hoses. I had a leak in the membrane of the vapouriser . To get it stable as I said went back to original cooling configuration. I have replaced all hoses cap thermostat pump radiator and heater matrix the system holds pressure - I get hot air through the heater above 1500 revs and below that cold.
The car over heats if driven hard and yet when taken down cools down I am at a loss oh and I just did a compression test again this afternoon and getting 165 across the board on the right bank on the left 165 165 170 160 so really no issues there
Question would the comp test hold up if a liner was slipping oh and no coolant loss
HELP I am going nuts

If the heater only gets hot above 1500 rpm, you have a circulation problem IMO. If your new pump had a plastic impeller, with all your overheats it may have started slipping on the shaft. Otherwise are you sure all the plumbing is correct as per original?

Air in the system

The new pump had the metal impeller have had it off twice to check and it is ok the way I am filling it I am removing all the air I back fill the heater matrix then go through the whole procedure as per rave yesterday ran it for nealry an hour at 1500 rpm with cap off no air bubble what so ever also redid the sniff test absolutely clear slowly going mad well madder as Data always says you have to be mad to run a v8 when there is a perfectly good diesel equivalent trying a few things today but not sure what else to do