I have a lovely 2006 XC70 in great condition. Transmission is a Aisin TF-80sc. I am experiencing what I am told is most likely shudder due to bad TC lock-up.
This shudder happens at around 1700 1800 rpm at 22 mph and 40 mph.
No codes have been found and gear changes are fine.
What I don't understand is when the shudder occurs, if I hold that RPM the shudder continues for however long I wait. If I increase OR decrease the RPM say up to 2000 or 1600 the shudder stops instantly. If I hold the RPM while its shuddering there is no visible flutter of the tachometer. My question is 4 parts. (1) If its TC lock-up slip why would a slight increase in RPM stop it and not just make it worse?
(2) If it is TC lock-up could an ATF drain (replace) fix it?
(3) If it is slipping would I expect to see some sort of fault code stored?
(4) Any ideas on other possibilities?

All or any help would be gratefully received.

Lucky Pete (or maybe not)