Bicycle Mechanics - Slipping/Skipping?

I ride a fixed-gear bicycle. Earlier tonight while I attempted a skid, it felt as if the drivetrain skipped a beat and gave out on me. I put resistance on the forward motion and it felt as if it just gave way for a moment. I rode it for a few more minutes and when I start from a stop and mount onto the bicycle, the drivetrain will just skip or slip for a moment until I start moving forward. Something must be loose or stripped, but I am not sure where to start or look. I'll probably be going to the LBS in the morning to get it checked out, but maybe you guys could help pinpoint or narrow down the possibilities as to what might be wrong. Any help would be greatly appreciated, thanks.

It's highly likely a loose cog/lockring interface. I would using brakes to stop until you get it into the shop. If you skid on it more you risk stripping out the hub. That will be a rather expensive repair.

That will be a rather expensive repair

So I went the LBS the following day and they tightened the lockring and that solved problem of the slipping. It was fine up until tonight where I let my friend of a heavier stature attempt to ride my bike. As he mounted the bike and started off, the drivetrain slipped. I'm guessing his larger stature can put more torque on the drivetrain even just starting off from a stop to loosen the lockring than what I can with a skid. I weigh maybe 100lbs, he weighs probably upwards of 170lbs, but we are the same height.

After the initial tightening, I asked the guy at the LBS if he had noticed if the hubs had any signs of stripping, but he said he didn't notice anything. I asked if I should just replace the lockring, but he said it seems fine and to maybe use some loctite. After this second slippage and loosening of the lockring (assuming it is the lockring again) should I just go ahead and purchase a new lockring?

If it is any help, the bike is a stock Mercier Kilo TT which I have heard to have a reputation of not having the greatest stock lockring.

You have to torque the sprocket tighter than riding will, otherwise one day extra torque will pull it forward off the lockring and allow the kind of back and forth you describe.

If you have brakes, find a steep hill and hard climb it maybe even against some braking to really pull the sprocket tight, then bring the lockring up against it. Or better yet, let your heavier friend set it for you the same way before setting the lock ring.

If it is any help, the bike is a stock Mercier Kilo TT which I have heard to have a reputation of not having the greatest stock lockring.Apparently this bike has a flip-flop hub. Which hub is this? Does the fixed-gear side have a reverse-threaded lockring?

If you feel it slipping in the forward direction, you must stop and tighten up the lockring before going in reverse. Going back and forth repeatedly will just loosen the lockring and cog more and more.