Feeling of suddenly missing chain in bike while pedalling I can't pinpoint the problem with my bike, but the feeling is that it suddenly feels like I have no chains, and then it comes back, like it's "skipping". 

UPDATE: From the comments, I think the proper description is that it suddenly stops engaging from the freewheel. Usually it happens when I'm vigorously pedalling, or when I pedal in reverse. There's also a snapping sound when it happens. And then it returns to normal after a while.
There are three possibilities that I can think of to start with:


Your chain is worn and no longer fits the cogs on the cassette (or the single cog on the hub of a bike with internal gears or a single-speed bike).

In this case the problem would usually be worse on higher gears (the smaller cogs in the rear) and under load. It probably wouldn't happen if you were pedaling gently on the flat for example.

If you have a ruler it is fairly easy to check for chain wear. Just measure out 12 inches along the top of the chain, starting from the center of one roller. On a new chain the center of another roller will line up with the 12" mark exactly. As the chain wears it elongates and by the time that roller is at 12 1/8" the chain is worn out. Much beyond that and skipping can start. If the chain is very worn it is best to replace the chain and the cog(s).
Your freewheel is malfunctioning. Two things could be wrong, there could be damage in the freewheel – either broken pawls or rounded/broken teeth, or corrosion or dried out lubricants could be causing the pawls to stick. The photo below, from A New Recyclist shows the internals of a single speed hub.

In this case the problem would be most noticeable when you were starting to apply power, damaged pawls/teeth might also skip as power increased as well. If you have sticking pawls I would expect them to hold once they seated. Broken pawls might never seat, and broken teeth would mean that there would be small (much less than a revolution of the pedals) skips when the pawls land on damaged teeth. Sticking pawls could let you go several pedal revolutions before they catch.

You might also be able to diagnose this by listening as you coast. The sound of the freewheel should be a very regular fast clicking as the wheel spins. Then as you start to pedal there should be a consistent firm engagement, perhaps with a bit of click after a moment of silence as the cog catches up with the hub.


Your shift mechanism is badly adjusted or you are shifting inaccurately.

If the shifters are out of adjustment you may find that shifts either happen spontaneously or that you will sometimes do a "half-shift" where the side plates of the chain will land riding on the teeth – rather than on the sides of the teeth with the rollers engaging the teeth. That's not a very stable configuration and usually the chain will drop into a gear, but until it does the pedals will spin freely.