I seriously doubt it has anything to do with the chain slipping on the Surly cog even when using a tensioner pushing down - unless your chain is stupidly long or you don't have enough spring tension on the tensioner.
I'd say its more likely to be the freehub slipping/failing.
Well, I tried a half link & can't get the KNC Snap link to fit now as it's a fraction too short. So am trying a 17t Cog in place of the 16t.
Hope it's not the Freehub? Seems OK, latches OK on freewheel. I'll try the cog & maybe the BB Tensioner, but if all else fails I'm going to put gears on!!...(at least on the back...)
Had a similar problem on one of my bikes where I thought the freehub was slipping. I was told if it's the freehub it'll be relatively quiet as it slips. If it's the chain/ sprocket/ chainring it'll be noisey...My turned out to be a chainring.