You probably need to change your timing chain. It is not the age but the wear that counts. Worth buying a Park chain-wear tool (not that expensive) especially if you have several bikes. If you change your chain when it is worn to the limit of the checking tool it will give your cassette and chainrings a longer life and in the long run save you money. 

From the pic it looks as if you are able to create the problem without a stoker, so if it is the timing chain that is jumping you'll see a change in phase between the cranks as the front set advance relative to the rear set.

Can you mount the go pro to examine the front chain rings whilst you induce the problem? The way you describe the problem makes it sound like how front chain ring wear shows itself when new sprockets and chain are fitted.

Thanks.

I'm pretty sure the timing chain isn't jumping as the cranks are still in phase.  I'll check the wear on the timing chain (I do have a chain wear tool).  It could be timing chain rings so I'll set up the GoPro on that side to see what is happening.

I did do a GoPro video of the stoker driveside chainrings and there doesn't appear to be any chain skip.  I put a photo in one of the other posts but it is a bit blurry.  What is obvious is that the cranks jump in a couple of frames but the chain still seems to be on the chanring..  The chainrings are brand new. 

Here's a picture of the driveside chainrings at the point of jumping.

Just a long shot. Does this only happen when you are on the 11 sprocket? I had a problem like this with a new Connex Chain and a new cassette but only on the 11 sprocket. The problem was the connex joining link. It has a bulge on one side. That bulge needs to be on the side away from the Axel. If the joining link is put in with that bulge nearer the Axel the bulge fouls the cassette body between the 11 sprocket and the next sprocket lifting the chain up a bit and causing it to skip.

The sudden arrival of slack in the chain is obvious in your pictures from the wave in the return run to the rear mech tension arm lower end so it isn't timing chain slip. Are you sure the freehub body ratchet or for campag like hubs the ratchet inside the hub isn't giving way under load? We have had the outer bearing tracks break off in XT freehub bodies and this gave a rachet jump under load.

With new rings have you rechecked all chainring bolts are torqued up and loctited, plus are rings seated flush against spider arms?Lastly did you change b bkt?  I had a royce ti axle that flexed too much under pressure on timing chain side, didnt make timing chain jump but caused intermittent jump on drive chain as in pics above. Ladtly any chance you have fitted joining link off a different spec system?  Hope you get it sorted.

Hard to see what is happening from that pic but from the chain reaction I'd say it looks like teeth are being jumped. 

The pics make it look like you are on the middle of the block and on the middle ring with a decent capacity derailleur, so one thing you could try is to set you chain length to 3x the number of teeth on your middle ring. With a 38 ring this would be 114 links ie less than the oft sold 116 length chain. For more teeth and links you might need to take some off the old chain to get the numbers right. Now you have an integer multiple of links to teeth you can mark on the ring the meshing point of the joining link and every three crank turns it should come back to the same place. If after you've provoked the problem the cranks have advanced relative to the joining link you know it is skipping on the front chain ring. I'd definitely check that each link pivots freely and that there aren't any twisted or defective side plates  

I don't know GoPro but there were some good slo mo apps for the iPhone which might be worth a go for better detail from the frame by frame pics. 

All the other comments by others above make sense for checking and I'd also check your cassette lock ring is done up tight and the sprockets are all snug and square on the freehub body.  Good luck :-)

Thanks for all the suggestions - it looks like the problem has been resolved! It was chain slip on the front drive chainrings, as suggested by a few people.

I checked everything that had been recommended and all was fine.  However, I couldn't replicate the problem on the big or small rings, only on the middle so I looked more closely at the middle chanring and noticed some scarring on the top of the spider arms above the chainring bolts.  It appears that the rounded outer edges of the spider arms were raising the chain just enough to cause it to slip over the teeth under load.  So I filed a millimetre or so off the rounded outer edges to make them flat, and no slip (and things are much quieter).

Quite why it did it only when I changed the chain and chainring I can only guess (old chain wore down to fit? old chain wasn't quite as deep??) given the new chainring was 38T, the same as the old one.  I know 38T on a 130BCD spider is the minimum so this is probably why most 130BCD spiders are 52/39 rather than our 50/38!

Once again, thanks for the suggestions.

Neil.