My crank slips I think

Your freewheel is failing - the pawls are not grabbing immediately which allows the chain to move without the rear wheel being powered.

This will not resolve itself over time, it will get worse.

Your two solutions are

Temporary) try washing the freewheel with solvent to get the gunk out. Then let it dry and relube with light machine oil.

Longterm) Replace the freewheel mechanism with a new one. You will probably need a new chain at this time too.

My son and I have had direct experience with this (he is helping me write this). I realize this thread is very old, but I put this here in case others are finding it.

First, check the chain. If it is worn too much (they elongate over time) it won't grab the teeth.

Second, Check the chainring, Are the teeth sharp? If so, you need to replace it?

Third, check the teeth on the rear cog. Again, if the are sharp, you need to replace it.