Shimano drivechain help required

[tnike]

I have a 2011 specialized secteur that came with sora groupset on it.
I always keep my bike properly cleaned and lubed.
recently my chain started skipping round every so often on certain gears when I push hard through the pedals such as hill starting, but this would also happen randomly mid pedal strokes, say on the flat. I messed with cable tension and new cables etc but no joy and the cogs started to show signs of being damaged.

I checked with the chain wear indicator and it wasn't yet even worn to 0.75 but I made the decision to replace the 9 speed hyperglide hg50 cassette, 9 speed HG93 shimano chain and chain rings. I replaced like for like, however the problem is still there! with brand new compatible drive chain components too! I might add also that the skipping / tripping of the chain is happening on most cogs and appears not to skip onto other cogs when the problem occurs.
Checks I have now made are:
1. Hanger is perfect.
2. Dropouts are fine and wheel sits in dead straight every time.
3. pulley wheels on Tiagra rear mech are in good condition and rear mech is in good condition
4. no stiff links detected in chain
5. gears indexing well.
Any ideas appreciated before my new drivechain components get trashed!! I was thinking perhaps the FH-2200 freehub is worn but I've only been riding the bike for about 9 months and I've had similar freehubs last years longer. plus it is my understanding that freehubs are 'all or nothing' i.e. they completely malfunction and it is conclusive rather than partially malfunction under pressure like my problem - please correct me if I'm wrong.
Should I try a more expensive cassette? can you recommend one?
Cheers,
Tim