Derailleur/Gears Locks Up/ Freezes Up/Stalls Hey guys I have a specialized mtb with the old oval chainrings and lately it's been locking up. When I start pedalling "hard" or faster the crank stops turning and then jumps forward a bit and generally is not very pleasant. Please note the wheels keep turning fine. Has anyone had this problem before? It's happened to me on another, thankfully cheaper, bike that was stolen a year before I built my fixie (which gives me no such problems) a couple years ago.

To recap, the pedals stop turning and then jump forward, stalling in a way, when I start to pedal harder, like up a hill or just on flat but faster. How do I fix this?

Thanks for any help! I really love this bike!
This is most likely an issue with your bottom bracket.

To diagnose, remove your chain from the chainwheel so you can spin the cranks without the rest of the drivetrain being involved. 

The easiest way to do this is to use your left hand to pull the rear derailleur's cage towards the front of the bike, creating slack in the chain. Then with your right hand pull the chain off away from the bike. You don't want to drop it in between the crank and frame as it's likely to get jammed.

If the cranks spin freely, with no "bad spots" and without creating a "grainy" sound, then your issue isn't with your bottom bracket and is likely a damaged/worn chain or rear derailleur cogs.