Originally Posted by
bradtx
For a bicycle I wonder if the freehub or freewheel could be the cause. Have you spun up the rear wheel on a stand to watch what's happening when the crank arms are stationary?
Brad
This is a very good suggestion. Get the bike up on a repair stand and spin the rear wheel up pedaling by hand as fast as you can in your highest gear, then let the wheel coast. You could even backpedal a bit and watch what the chain does. It will give you an idea what is happening when you coast downhill. If the freehub has too much internal friction the top run of the chain will go slack. If it sticks, even momentarily you would know what happened
A freehub that makes little noise could easily have sticky pawls