Derailleur/Gears Locks Up/ Freezes Up/Stalls
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.