I've got a 250W electric strimmer with a commutator type motor. It is
running much more slowly than it should (I would guess a few hundred
instead of few thousand rpm). Both brushes spark as it runs.

I have checked rotor windings with a multimeter across the brushes,
rotating the rotor readings hover around 20-40 ohms.

There are no obvious high or low points on the commutator, and I have
polished the contacts and cleaned them and the gaps between with
methylated spirits.

The brushes are about 15mm long, and are held snug against the
commutator by springs

The field (stator) windings are also about 30 ohm each.

The rotor spins fairly freely, there is one point where it rubs
slightly, but only very slightly.

Can anyone suggest what might be wrong?

Regards,

Nigel