major tight spot on chain

[Simon.Pieman]

I'd get a new chain, it sounds like the o rings on those links have failed and the lube inside has gone and/or water has got in and it's corroded up. Very often the soft link on an otherwise good chain goes stiff for this reason because the o rings get damaged when you pound the rivets, then they fail.
Lubing the chain so that oil gets back inside the rollers can work, but invariably it's better to buy a new chain.