This is a problem with which I have been struggling in this model
(U6425-920) since very early after I bought it... The agitator will
spin intermittently and some rubber smell will be noticeable.

The brush roll is new (and without any hair within).
The flat belt (P/N 38528035) is new.
The V belt (P/N 38528034) was replaced about 4 months ago.

The manual says "The V-belt is designed to last for a long time and is
not normally replaced".

These are the only replaceble things I can think of.

How do I troubleshoot the problem?

Is it possible that the Flat belt and the V-belt must always be
replaced at the same time, to minimize the chance that one is damaging
the other?

Any ideas except for throwing out this vacuum cleaner and buying...
buying what actually? (nowadays everything is made cheap and
unreliable)

Thanks!
Sam