Temperature Guage Fluctuations

[Cubes]

Last year I had the coolant changed and radiator rodded with twin thermo's fitted.

It used to warm up to 1/2 on the temp guage really quickly and never move either cold hot or AC on the guage wouldn't budge.

NOW..

I recently had a problem where the car was overheating so I changed the thermostat with one that suits a VL, It turns out the the Temp is the same for the VL and the RB20DET (76.5degrees)

I put it in and it still overheated. So i flushed the coolant system with Mukout for 2 days and it fixed the problem.

Since then its been running too cool. It would reach half on a hot day or if the AC was on. On the open road it would then drop down to around 1/4 and some times even just above the C Line when coming down a hill etc.

I put it down to the Thermostat.

SO.. I bought a new Nissan Genuine Thermostat.

I sent the car in to NATRAD in Adelaide to get the top tank replaced as it was brittle and they did a radiator clean (20% blocked) block flush and replaced the thermostat with the one I provided.

BUT it still is doing the same thing.

I drove for 20km's this morning on 60-70km'/h around the seafront it didn't move above the C line.

So I idled and hit the ac. It still didn't really bring the temp up.

I the gave it heaps with the AC on and it brough it up to 1/2 on the temp guage.

Hit the open road on the way home and it dropped to just above the C again. Less than a 1/4.

It is still doing the same with the new thermostat as what it did with the old one.

Its not good for an engine to run to cold.

Any one have any Ideas?