I've just converted a mid 90's GT Outpost MTB with vertical dropouts to a single speed using a Surly Singleator, Surly 18t rear cog, a new Thorn 44t chainring and a new 3/32 KMC chain. I have the Singleator pushing up as recommended and the tension is good, but the chain is catching on the rear cog and skipping when any pressure is applied. As far as I can tell, the chainline is straight and the chain tension is good, but I have no idea why it is doing this. Is it possible that some combinations just will not mesh and that I might be better off using a different rear cog with less/more teeth? Any ideas here would be much appreciated, so thanks in advance...

The two things to check first off -

Make sure there aren't any stiff links in your chain.

Make sure there aren't any bent teeth on the cog.

yea check for bent stuff

Is the Surly cog 3/32 or 1/8?

Many thanks for those answers there, they're much appreciated. I will check all those things and hopefully I won't need to come back...Cheers all.