Strange intermittent overheating

I have a '93 Hilux Surf 2.4 with some odd symtoms:

Car's temp gauge occasionally shoots up from half way to just below the red (turn engine off at this point) within matter of seconds. In addition I am loosing coolant and have varying coolant levels in overflow (which occasionally leaks).
- Granted this points to head issues, however hear me out:

-There's no smoke
-No oil in water or vice versa
-No permanent power loss (jus occasional slow pull off)
-Overheats only on low revs
-Whenever car is run past 2000rpm temp gauge drops, again in seconds, back to half way!?!
-Again this is just an intermittent fault.

Don't know if this is related but symptoms occured just over a week after following were replaced with genuine brand new Toyota parts:

-Timing belt/fan belts/tensioners/water pump/thermostat/all fluids/all filters.

Was in devon over weekend and drove car around town (stopping and starting) in heat for about 1hr with no problems. Drove 170mile trip back last night with no issues until came off motorway and revs dropped.
Topped up water this morning and ran with cap off until warm and drove 10 miles to work in traffic with no issues.

I suspect a blockage or air lock of some sort in cooling system/rad which is causing back pressure into engine until revs are high enough to supply water pump with enough power to force through.

Really confused - can anyone offer any advice?

Cheers

Ryan