Yes, I've had a similar problem with 3rd gear just spinning under even the slightest load. The cassette came loose somehow and does not put enough pressure on the 3rd cog to snug up against it's spacers. There is no spline for 3rd gear to slide on to. Very bizarre design. Can't get the lock ring to tighten up enough to snug 3rd gear down so it won't spin. Contacted Radpower warranty department and waiting to hear back from them after I've had a professional bike mechanic try to fix it and he was not able to.