DC07 cutout?

We've had a DC07 (purple/blue with HEPA) for about 5 years, and around 3 years ago it starting cutting out after approx 5 mins use. I cleaned all servicable parts/filters to no end, and eventually called Dyson who diagnosed the fault and sent a new cyclone unit. After replacing this, everything was ok again.

Recently though, the same problem started happening again. After approx 5 mins, the thermal cutout kicks in, and the DC07 stops working until the motor cools down. I followed threads on this site, and thoroughly cleaned all filters again and the cyclone unit itself (fill with hot water etc). This did NOT fix the problem though. I decided just to order a new cyclone unit, which duly arrived and was fitted. Trouble is, the cutout is still happening, even with the new cyclone. I've cleaned all filters (inc HEPA), checked all pipes etc. Everything looks fine. After cleaning, suction was great, but the cutout is still happening.

I've followed various threads here, and the full cyclone clean seems to work for most, but not for me!

Any advice appreciated