I second most of what has been stated above. Your times seem fine. Don't forget "gentle agitation".
Or, as I know it, "move it around a bit".
See:
https://www.ilfordphoto.com/amfile/file/download/file_id/2012/product_id/711/
(Rapid Fix)
and
https://www.ilfordphoto.com/amfile/file/download/file_id/1957/product_id/743/
(MGIV RC)
-Doremus, the cached sheet you refer to is an earlier version but mostly small changes.
Reading what you have said about washing I suspect that the fixer is the most likely culprit. If you send me the batch number then I'll tell you how old it is (eg.41B020).
STORAGE: Full, unopened bottles of ILFORD RAPID FIXER concentrate stored in cool conditions, 5–20ºC (41–68ºF), will keep for two years.
Once opened use completely within six months and keep all bottles tightly sealed until used.
(Some people can store for longer and find all is OK but we have to be conservative, lower temperatures -never freezing- are better.)
Regards,
David
(technical@harmantechnology.com)