Losing heat on highway

[Magoo]

I am losing my heat at above 60mph-3000rpm, usually within 10 seconds of reaching that speed. If I drop back to 55 or less, it comes right back, within a few seconds. Engine temp doesn't seem to matter, 80 - 100C, still lose heat at higher speeds. I've been searching but have not found a thread that matches my symptoms. Ideas anyone?

Thanks