Chain jams between smallest and second smallest cog
 

I just replaced my chain (8 speeed SRAM) and cassette (8 speed SRAM) on an 86 Trek Elance 400. This bike is also equipped with a Shimano Light Action (originally paired to six-speed indexed shifters) rear derailleur and a set of Shimano 105 downtube shifters (eight speed indexed). My problem is that, occassionally, when the bike is on the large chainring (in front) and shifting from the smallest cog to the second smallest cog, the chain jams between the two cogs while trying to shift. This locks up the pedals (a rare problem I never want to encounter on the street). I was thinking that the chain is just a bit too wide, even though the whole set-up is 8-speed. I'm going to replace the chain with a narrower one (for safety), but, before I do this, I want to see if anyone has any ideas or suggestions of a fix. I'd prefer not to buy a new chain if I can help it. Thanks.

Tight link in chain?
Loose cassette lockring?
Bent tooth on cog?
Wrong chain?

Improperly set b-tension will do this.

you don't have a rack or fenders by chance? A bolt or protruding screw on the inside of the dropout can do this.

I'd say it's got to be the b-tension adjustment.... But, the OP said the bike was originally a 6 speed but is now an 8 speed. If they were spread the chain could be clipping the taper of the chainstay and jamming. I've had 126 mm spaced bikes where the chainstay protudes more to the inside than on most modern 130 spaced bikes.

zzyzx_xyzzy - that was it exactly. The cassette and chain are new and the rack's screw was just a hair too long - protruding into the space right above the rear cog... just barely. This was enough to rarely catch the chain. I replaced the screw with one that was a couple mm shorter. No more problem.

Everyone, thanks for the help. I was focused only on the cogs and that wasn't the problem.

That actually seems way more plausible than my explanation - at any rate the OP solved it with some common sense. Same problem arises with 5-6 speed conversion to 7sp+. Usually we just jam a spacer on the driveside to space it out. Depends on the bike/customer.