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Around once every other ride, when I start to apply power to the pedals there will be a loud bang and the cranks will slip round a little.
It is not the chain slipping, so does it sound like it is the bottom bracket, or more likely the freehub slipping?
Sounds like the chain slipping on worn cogs to me
Pawls in the freehub are slipping, if it's a hope hub or suitable aftermarket item then replacement pawls are available, if it's a shimano item then a new freehub is the order of the day unless you find a bikeshop with the suitable tool and patience to dismantle and clean the internals, lube and refit, this is most unlikely as a new freehub plus fitting will probably cost less then their labour costs for doing such a strip-down job.
My Race Face cranks did that eventually. Worn spline wonnit.
Unless theres a concurrent sharp pain in your knee?
A dying freehub gives precisely those symptoms and is usually the cause.