Shimano st5700 shifters

Hey all. I'm having a problem with the right hand shifter/rear gears that I haven't experienced before. It comes down to this, starting from the smallest sprocket the gears only shift down (to the lower gear/larger sprocket) with a strong push of the lever. If I slow it down it seems that the gear doesn't change on the click but just past it. Surely this can't be right, the whole point of indexing is that gears change on the click. The problem with going past the click is that it's easy to do a second click as the 5700's have the quick shift feature (2 gear changes in one stroke). This feature works fine by the way, consistent with the strong push being needed to change the gears. I think I've ruled everything out bar the shifter itself. New chain on, changed all the cabling, tried new cassette, same issue remains. I'm using a sram 1051 chain and I've read elsewhere that a Shimano would work better but I've been at this a while and never had issues with sram chains like this. Really does feel like it's in the shifter, which is pretty old. Does anyone have experience of how these wear out and fail? Is what I'm describing consistent with shifter failure? one other strange thing that happens intermittently is that when I press all the way through for the quick shift sometimes it performs 3 clicks rather than 2. This leads me further to think that there might be something failing in the shifter mechanism. Any thoughts would be greatly appreciated.