2 Strokes Down Under

Hi all,
I need advice please ! Been struggling to sort out an annoying overheating problem on my RZ250 84 model.
I have bled & replaced coolant twice and I have put the cold thermostat in a saucepan of boiling water and it did open, (dunno at what temp as didnt have a high temp thermometre).
I have also replaced the radiator cap a while ago.
Then recently I removed the water pump cover just to have a quick look the pump impeller was intact and spinning freely, which it was.
So after all this the bike still overheats whilst riding in traffic.
I need to fix it ! What else could it be ? Can anyone pls offer advice ??

Hold on... something isnt right with this bit:

"Then recently I removed the water pump cover just to have a quick look the pump impeller was intact and spinning freely, which it was."

The pump is driven by a gear and will not spin freely when spun by hand... This sounds like your problem...

(also flush your radiator when you have this stuff apart, it will help, and you can also grab a bigger rad - 40mm version - off ebay.

RZs run very hot when not on the move. The rad is way too small so as John said get a bigger rad or don't sit in traffic.

Defo... RZ std rad was 450ml, great for Northern Europe... not for Aus. 40ml rads ate 750ml, same capacity as the old LC rad. Who knows why Yamaha downgraded the rad for the YPVS... mental.

THankyou to Jon W and EPApolice - very interesting to hear that standard RZ radiators are really too small capacity, never knew that. Also i didnt know other RZ's often run hot.........I will think about getting a larger rad, and i will flush out the rad as well.

We had an Oz delivered RZ350 for a few years which never overheated in any temp/traffic conditions, so I'm sceptical that it's down to the standard rad being too small. This bike does 'pump' coolant out of the overflow bottle cap sometimes. That any clue? (I thought it might be prob with the head gasket causing pressurisation, but there's no coolant in the oil and no oil in the coolant).
-Jools