Skipping under load with new chain and cassette
I'm riding an 09 Genesis Vapour, original short cage 105 rear mech, Sora compact. I've been using it as a wet weather bike/commuter with a 25-12 cassette, but when I wrecked my MTB recently I put the Cross tyres back on and changed the cassette to a Shimano 28-12 so I could go XC on some steeper tracks.
The existing chain (2 months old, not worn) skipped a bit on a few gears, so I replaced it, but no joy. Now the cassette and chain both have about 20km on them. While the highest and lowest gears are fine, all the gears in the middle of the range skip badly under load. I've replaced the rear gear cable, stripped and cleaned the rear mech jockey wheels and as far as I can tell, the indexing is OK and if I put the old 25-12 cassette back on the problem goes away, even with the new chain.
My thinking is that the 105 short cage mech can't handle 28-12, even though it has no problem in getting to the innermost cog. However the guy in the (Edinburgh Bike Co-op) shop reckoned it would be fine, but the 32-12 wouldn't. This seems to be the common reasoning elsewhere too.
Any suggestions as to what to try next? Chain length? Derailleur hanger angle adjustment? New jockey wheels?