[wunhunglo]

OK so it looks like a gas problem then. The disturbing thing is the bit about the upper front handguard going flying. Is it leaking gas at the gas port??? Have you tried taking the handguards off & spraying some light oil around the gas port area then firing it, with the guards off, preferably with someone watching to see if the oil gets blown all over the place.
I assume (Ass-U-Me) that you have thoroughly stripped it, cleaned & inspected the gas piston (& it's spring), the gas plug, gas sleeve (have you adjusted this?) and checked that the push rod (tappet) is not bent and binding when operating. Also that the breech block is not binding anywhere and is properly lubed. Having said all that, if you've done the basic routine cleaning I would concentrate on the gas sleeve adjustment first before delving too deep.