you gotta make sure the cog is tight. it might be that you backpedalled hard when the lockring was a bit loose, and your cog is tightened backwards against the lockring. in this case, tightening down the lockring will only tighten against the cog, but the cog will slide forward as soon as you put enough forward force into the pedal.

loosen the lockring, take a chainwhip to the cog and make sure it's tightened down hard against the hub. you can do this on the bike by applying the brakes and standing on the pedal. then tighten the lockring all the way down in the other direction.

i hope the issue is not deeper than that...