Recurring Air lock - Sigma Cooling

[krisfl]

Howdo Hive mind. Hope you're all well as can be.

I have a recurring airlock on my Ti-VCT sigma. Basically after a good run the expansion tank is full to the brim and the engine temps climb. Cracking the bleed bolt on top of the Rad with the tank cap off sorts everything out. Till the next run... It does this with the heater on or off. Makes no difference...

So far I have:

 - Bled x3 myself.
 - System checked and bled by a Seven Specialist.
 - Checked all hoses and connections everywhere, inc heater. All look and feel new.
 - Checked the infamous blue T piece

Next
 - I'm going to swap the expansion tank cap (it looks new, but its cheap so...)
 - Swap out the rad (possible pin hole and some suspicious foil tape from the previous owner over the top and bottom bolts leading me and the specialist to be suspicious)

But anything else I should be checking first? Or after? What could I be missing?

Ta muchly.