Bicycle Mechanics - Cranks slipping - trying to work out why

A few weeks ago the cranks on my hybrid commuter bike (3x8 speed Shimano, 28-38-48 x 11-31) started slipping, by which I mean that sometimes when pushing with moderate to hard force they would suddely rotate forward through maybe 60 degrees with little reistance, accompanied by a clicking/crunching noise.

I did the following things, with no change:
- checked alignment of rear deraiilleur
- replaced chain with a new Shimano HG50 8 speed
- replaced cassette with a new Shimano HG50 12-25 8 speed

The problem is getting steadily more frequent but is still intermittent enough to make it hard to diagnose.

Does anybody have any suggestions as to what the problem might be? I have tried to look and see butwith no luck, as it only happens when under pressure that means while I'm riding, in which case it's inadvisable to be staring down at your drive train rather than at the road and cars around you. My best guess is that maybe the freehub's ratchet mechanism is has developed a fault, but I don't even know whether that ever happens.

If all else fails I'll take it to a bike shop but I'd rather fix it myself because that'll mean less time without the bike.

Would go for the freehub is failing, replace this, and see if it resolves it.

If it's a few seasons old then the ratchet pawls inside the freehub may be just getting gummed up. Strip the hub and remove the freehub and flush it in solvent followed by a solvent and thick oil mixture. Dunk it multiple times and spin the freehub in the solvent to work it in and the gunk out. Let drain well then dribble in a mix of thick oil and solvent that is thin enough to easily run in through the small joints between the two moving parts. Allow to drain for a couple of hours after working in the lube mixture and then re-assemble the whole hub and axle.

Over the next day or two the solvent will evaporate away and the hub will get a little quieter. But you can ride it right away and it should be fine if this was the trouble. And since you've already replaced the chain and cassete I can't see what else it would be.

Does it do this with the other chain-rings?
I had these exact same symptoms. -Intermittent, while under variable load. I would get 30-60 degrees of rotation when it slipped.
It really seemed to me that the freehub was missing/skipping the ratcheting while under load. The root issue was the front chainring was worn out.

Thanks for all the replies.
I tried it on the other chain rings and identified the slipping only happened on the middle one, so I replaced the crankset. The middle chain-ring was looking very worn.
The problem is now fixed.