05 WRX overheating

Been having intermittent overheating issues on my WRX.

It's a well used one with 160k miles.

I do the same journey every day, sometimes the temp goes to the 2/3 mark on the guage sometimes it stays on 1/3.

Driving style really doesn't make any difference.

Yesterday, I did a full cooling system refresh. New coolant, water pump/thermostat, timing belt, all idlers and pulleys.

First drive home it was absolutely fine, about 30 min drive.

Then, the next drive, it overheated. I stopped as soon as I saw the guage and I could hear coolant bubbling.

How the hell does this happen?

Right now I'm doing a lengthy coolant bleed with the big funnel. No bubbles coming out at all. Heater is on and car warms up consistently. The fans turn on eventually, and the temp keeps rising! It doesn't drop so the fans stay on. Temps got to half way mark and I switched it off.

Top of the radiator and hose is very hot, bottom hose is cool. I flushed all hoses and the radiator as part of the work.

The hell is going on??