Don't use the cheap lockring tool. The Park HCW-5 ($15.00) works fine. I had the same problem with my cog coming loose because I only had that 'hook' lockring tool (which is on one end of the Park tool). Couldn't get it tight enough because it kept slipping and busting my knuckles. The bike shops around here use the very expensive plier-type tool from VAR ($75.00).

Or just take it to your bike shop. That lockring needs to be really tight on there.