Borg warner 66 juddering when hot

[V8cortina]

I have a borg warner 66 auto box in my RV8 powered cortina, ever since the engine and box went in, I've had a juddering from the drive train under acceleration in 1st and 2nd but only when hot, its fine when cold.

The ATF is still nice and clean so the box isn't burning out but it does feel like its slipping when the juddering happens. Any ideas?

Bigger cooler? kickdown cable adjustment? How do I go about adjusting the bands?

Thanks in advance