Chain slips/jumps occasionally (with all new parts) Several months ago I replaced all gearing parts and switched from an 8 speed to a 10 speed system. All parts were brand new.

After replacing I noticed the chain slipping occasionally. Though I drive for months now, the phenomenon first happened days after replacing all parts.

I had the derailleur cable tightened after several weeks and once more after several months (it was tight enough this time).

It happens


on the cassette (whatever it is)
without heavy load (not accelerating from stand, riding uphill, putting only slight pressure to drive slow)
with moderate and heavy force too
without bumpy road
not during shifting the gears
with front gear 2 and rear gear 3 - 8, front gear 3 and rear gear 8 - 10 (I rarely drive lower gears, can't say whether it happens or not)
at most once within 50 km
with the same rate after more than 1000 km
with the rear derailleur Dyna-Sys switch on and off


I can't


reproduce it (at will)
say whether it happens in all gear combinations or not
say what exactly happens (chain trying to change the gear or messing things up somehow on the gear) since I can't see it


I had two thoughts:


the joined link could be a little bit stiff (but I can't see any stiffness when spinning the chain in stand)
the derailleur is not properly justified (I had it realigned after several weeks and months, though must admit, I'm not a professional in doing this)


The chain is properly and regularly lubed.

Should I still assume a stiff link or the derailleur being misaligned? What could be another reason?

And no, this was not a mixed setup. I read this question on a new bike too, I already addressed the mentioned issue, I think.

Setup:


FC-T521 3 speed crankset
XT 10 speed cassette CS-M771-10
XT FD-T780-6 66-69° 3/10 speed front derailleur
CN-HG95 10 speed chain
Shadow Plus RD-M786 10 speed rear derailleur
a theory: you might have too much friction between the derailleur cable and its housing. such friction could prevent the chain from going to the intended place when you shift; when the friction subsides, the cable would move and the chain would slip into place. 

apply some lube to the cable and slide the housing back and forth to spread the lube along.  this could help if my theory is correct.