HELP! 93 500SEL low power problem

[Mercedes500sel]

My 93 Mercedes 500SEL has the low power problem in cold or hot engine, the exhaust smell like gas, idle is very rough, very slow in uphill situation.

I changed the following parts:
1. All new plugs.
2. All new plug wires.
3. New Rotors and Caps.
4. New air filter.

After changed all the tune up parts, the car still has the same problem.
Changed the fuel filter, problem disappear, and I thought the problem was fixed. Good power and very smooth, okay for about 1.5 months(1500 miles),
passed the smog test .

Now, the car has the same problem, very low power, rough idle, very bad smell from the exhaust. Changed the fuel filter again, still has the same low power problem. Please advice and help. Thank you!