Modded folding bike and strange chain skipping on most gears I've a folding Tern Link D8 that I've started customize by replacing some parts...what is driving me nuts is a strange chain skipping that happens on some gears and only when I apply some force (starting from moderate hill)

On the drivetrain I've replaced:


crankset ( FSA Carbon Pro Team Issue ) - USED
bottom bracket ( fsa platinum Isis drive) - NEW
cassette (Shimano XTR CS-M950 8 speed cassette 12-30T ) - USED
rear derailleur (  Shimano XTR M950 8 speed ) - USED
Sram gripshift with the right index ratio for Shimano - NEW
Shimano chain for 8 speed - SOMEWHAT USED


What I've tried without any noticeable difference until now:


Chain tension: I've removed or added a couple of links
Adjusted the rear derailleur as per manual (hanger alignment and L/H-screws) 
checked for stiff links
switched with the original rear derailleur ( Neos )
checked the cassette teeth...there is some wear but plenty of life here


The skipping is happening without changing gear, and seems a sort of "chain that jump on the chainring" feel.
On the smallest chainring usually it wont happen...I've made a video to better understand what kind of skipping is, but even in slowmotion I can really grasp what happens.


                
            
slow motion


                
            
original

Can you give me some suggestions?

SOLVED: the culprit were the worn teeth on the cassette: look at the pic below, I've highlighted the increased gap that cause chain to skip

green=still good
red= too worn
With reference to the new/used bits of your question, note that using worn consumables (chain, cassette etc.) on a bike is never a good idea, largely because you're in the dark as regards how much life is left in them. So my best guess is that the chain or maybe both chain and the cassette needs replacing for new parts.

Assuming that these parts are actually ok, then the problem becomes one of poor mechanics. Two things spring to mind:

(1) the chain you used as a replacement is the wrong length, and/or
(2) you did not adjust the derailleurs correctly after your chain/cassette changes.

And are you sure you have fitted things to this bike which are compatible with those you removed?