In reply to Yanis Nayu:
The first thing that came to mind (having worked on a good few bikes) when you described this symptom is a new chain not matching a worn chainring. You say the big chainring was changed - is the slippage occuring when you are in the big ring or just when in the middle and inner (assuming 3 rings).
It seems as if you had this problem before the work was done as you say that changing the parts 'hasn't solved it'. Did someone else do the work and, if so, why did they say why they only changed the big ring (same question to you if it was you that did it).
It's often the middle ring that's worn as many people use that the most, sometimes to the extent of never using other rings. Maybe you only use the outer ring so that's why it was changed, in which case I can only suggest, as others have, that a stiff link is worth looking for.
Edit: seen you posted as I was writing this and have answered some of my q's. Would suggest taking it back to the mechanic if you are not happy. Don't know about pedalling backwards being stiff is this with you sitting on the bike (i.e. pedalling with your feet) or just spinning the cranks backwards by hand?
One of those cases in which it would be better to see the bike; hard to diagnose on a forum but keep us all informed of what happens.
Post edited at 08:12