Big chain slip issue - can anyone help?

Hi all, I've got an older GT mountain bike, I'm running a single ring at the front (innermost/smallest ring) with an 8 speed cassette on the rear. The gears work great from 5-1 (middle of the cassette to the top/largest sized cog), but in 6th, 7th and 8th, the chain/cranks jerk with an immense clatter at the slightest hint of tension put on the pedals. A couple of things:

• Cassette and freehub splines/body etc, also the sprockets, are in good shape and clean
  
• I'm not running a chain guide of any kind at the front, but the issue seems to be at the rear
  
• When clattering, the cranks can spin but the chain at the back does not spin at all, very odd
  
• 2 clean, working mechs have been fitted - same problem
  
• Shifter works, brand new cable fitted
  
• In the jerky gears, when you backpedal, the chain brings the rear mech with it, rather than spinning around the jockey wheels. This of course extends the mech forward and sags the chain, before jamming up or throwing the chain.
  
  

I've tried most things, mech's adjusted correctly etc. It's driving me nuts! Thanks in advance

Is it a triple or double chainset? Does it happen when you're not cross chaining and using the big ring at the front?

^ hmmm: chainline...

Triple. The other 2 chainrings throw the chain off as soon as you rotate the crank, in any gear at the back. Chainline doesn't look great, but there's plenty of pics of 1x8, 1x9 bikes without guides or anything. Can't see where I'm going wrong

But 1x8 setups would have full sized teeth on the chainring, would be setup so the chainline is centre in the middle of the cassette and the chain won't be rubbing against the pins and ramps of the next chainring when on the smaller cogs.

If you want to run a 1x8 setup, put the chainring in the middle position if possible, but you might find your chainset is riveted, not bolted, so won't be possible without changing your crank. If changing your crank you'll also need a different length Bottom Bracket to get the chainline spot on in the middle of the cassette. Velosolo may have a full toothed chainring in your required bcd.

Is the outer ring still there?

Thanks for the replies folks, she's going full singlespeed