Scott Mountain bike. Problem with pedaling Last 2 runs out my bike I have as suffered from what I can only describe as the pedal free wheeling ie pressure put on pedal does not drive the bike, but after a couple of turns it then drive the bike forward. New cassette and chain installed by bike shop 2 years ago, only used once a week but in a fairly steep inclined countryside on short outings ie mountains. advice appreciated.
This can happen easily when a new chain is installed with a cassette that is worn, but you would likely notice that it's the chain skipping in this case.

It sounds like the culprit is the engagement system inside of the hub of your rear wheel. The engagement system has teeth that lock when you pedal, and if too many of these teeth become worn you won't get any power transfer, it sounds like this is your most likely culprit. I've had this happen before, and it tends to only get worse. If it seems like this is what's happening, take the rear wheel into the shop and see what they can do for you.