i took the car to MB dealer (only one official here in HK)
and they checked for 4 days and couldnt find out anything wrong.

they just said the battery water ratio is not suitable but dont know if it's the cause.

also, my car had strange sound (bon bon bon, like farting) at the boot bottom when I stopped the car after I accelerated to 3500rpm (around 160km, or 60km uphill). it is due to the exhaust pipe. they reposition it and it's now OK.