OK, aspiring bicycle mechanics, here is a problem I've never had before:
I own a pedicab. It is a large, passenger-carrying tricycle. All the of the forward drivetrain components (pedals, crankset, 7 spd. freewheel, derailleur) are identical to your average mountain bike. Since it is a tricycle, rather than a bicycle, the freewheel is not connected directly to a rear wheel hub. Instead it is joined to a single speed cog with a chain leading to the rear axle. You can see a picture of the mid-drive here: http://wheelgoods.com/store/product995.html Something like having a flip-flop hub for your fixie.
Anyways, my problem is that I have been experiencing intermittent slippage somewhere in the drive train. I will push the pedals and at a certain point in the revolution, the pedal will momentarily "go soft" before catching and applying force again. I understand chain skip. It is not either of the chains skipping. It is not any sort ghost shifting on the part of the derailleur. I checked the pedals/crankset/BB for any sort of play, but they are solid. The problem occurs maybe 25% of the time. When it occurs, it occurs in all gear combos. It doesn't seem to be related to applying a lot of force as it occurs during easy pedaling as well.
My best guess is that something is wrong with the freewheel. It is a regular old Shimano 7 spd freewheel. It seems like the chain rings, chain, and freewheel cogs are all married perfectly, but that the cassette isn't always transferring the force to the mid-drive. I've replaced cassettes and freewheels due to worn teeth, but I've never thought much about their internal workings. Is it possible for the ratcheting mechanism to fail? A sticky pawl perhaps? Ideas?