BostonFixed's method will fix your problem, Andy. 30 minutes is most likely more time than you'll need to spin that cog on as tight as it will go, I'd say 10 would be a lot of time to spend spinning it on. Your problem is caused by the cog not being spun on as tight as possible, then the lockring being threaded on only as tight as the "loose" cog will allow it to go. So there's some play in the cog, it spins forwards and spins off a little bit, but is stopped by the hub or the lockring. Spin the cog on, then retighten the lockring and you'll be fine.

The grease isn't the problem, the loose cog is. This is a very common problem, and is asked at least once a month, it seems. It's always been solved via BostonFixed's advice. If he's wrong this time, I'll pay for the plane ticket so you can fly out here and kick me in the junk, while pushing me into a busy intersection on my fixed gear, while I yell "Get out of my way you fools, I have no brakes!!". Sound fair?