Dear all,

Could you please help me identify the problems with my 8 speed shimano deore bike shifting system?
The chain drops down to the smaller cog suddenly , it is very often, another problem is when I pedale back, the rear derailleur automatically downshifts the chain to bigger cogs, and if I pedale normal forward, the chain will be forced back to the the position set by shifter on handlebar again, but as I like back pedaling, it is really a big problem, because in this situation, the chain will be popped off, or get stuck in the pulley.

I checked the alighment of pulley, looks normal, I checked the housing of shift cable,very smooth, so I guess something is wrong with derailleur.