When I press on pedal, intermittent loss of power Recently, when I cycled my bike the pedals would jump when I pushed down on them. I brought it to a shop who replaced the cassette, the chain, the two crank arms and probably the crankset. I have also had the rear gears replaced.

All great for about 15k, till the pedals started to jump again, when I pushed down. Thought it was sth to do with freewheeling/backpedalling when coming up to lights (as is particuarly bad when I stop/start) but that is not always the case. Also, the pedals spin when I push the bike forward - again, this is intermittent.

Have had it into the shop about 5 times, to no avail. Am lost. Bike is only about 7 years old. 
Anyone able to help please?!?!!
This sounds like there's a problem with your rear hub's freehub body (assuming you're running a cassette). The pawls get messed up sometimes and cause a jump when you're pedaling. If this is the case, you can try to replace the freehub body, which requires a bit of work. You basically have to take the axle out of the hub and get a 10mm wrench down there (most common method).