Hi guys,

Not sure about this one as there is probably more than one issue. About 6 months or so ago my old shimano (original on bike) snapped and basically was worn. So I replaced it with a SRAM PC951 chain and up until about a week ago had no problems. Made sure I kept it clean and lubricated. about two weeks ago the chain snapped and so I replaced a link with the spare ones I had left over when first fitted. The, last week the chain snapped again and so I replaced two extra links as the chain was a bit stiff. Checked that the chain links were fine and carried on.
I normally use the middle chainring and mostly as where I live is farily flat, so I have it on the lowest cog on the cassette. Every now and then when pedaling reasonably hard the chain seems to clip. If I move it up a cog the slipping stops.
So I checked the rear cassette and the chain seems to sit right on the teeth, however the middle cog they do not.

I'm thinking a few things now. Because I replaced two or three links for new ones on a reasonably worn chain this has caused a problem because most the links are stretched. Or, the middle chainring is worn and needs replacing, or the rear cassette needs replacing.
Failing that all three which is not the option I'd like to consider...

I'm just stumped why it is only slipping in the smallest cog??