Thanks a lot for replies,guys.
I have replaced a catalytic converter with another used one with 75kml on it. It made no improvement. But I am sure that my caps and rotors should be replaced because they are old too. I have them cleaned once already. Also I hear a slight clicking from one of the ignition coils. MB Doc suggest checking them too. I just noticed clicking from the coil today. Maybe that is the cause. Is there a way to check coils? It's funny but the car always had that low power than hot problem when I bought it 2 years ago.

Timing was checked twice by removing top covers and inserting pins into camshafts aligning vibration damper to 45 mark. My mechanic says timing is off but all marks line up. It's a strange problem.

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1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod