Crank is slipping freely occassionally when pedaling; occasional "clonk" sound
More information would be helpful. What kind of bicycle? How old is it?

Assuming the bike has a freewheel or a freehub, there are a couple of things you can check.

For example, if the bicycle has a freewheel (in other words, if it is older), it may need lubrication inside the freewheel. Automatic Transmission Fluid (ATF) is a great lube for that application, or you can buy various oils. I would advise against using WD-40 for lubricating a freewheel.

A similar kind of skip/slip can happen occasionally (say, once in a few hundred miles of riding) with a freehub, and it doesn't necessarily mean anything is wrong. However if it is more frequent, you may want to service the hub, or have your local bicycle shop service it.

If the freewheel/freehub is not the source of the problem, the drivetrain may be worn. The chain can skip, or jolt forward, if the cogs in the rear are worn excessively.