$2500+ in parts and still rough idle

Hi everyone!

I have a 93 500SEL (w140, m119) with about 90k miles. It developed a rough idle and was missing while accelerating a while back. Here is what I replaced to date (thinking I had to eliminated these things anyway before I went anywhere further):
1. New wiring harness
2. New air mass sensor
3. New spark plug wires
4. New Bosch spark plugs
5. New distributor caps and rotors
6. New coolant temp sensor

Had some improvement, but still rough idle (like its missing, but not all the time - rather occasional, once every 4-5 seconds) and it misses when accelerating in the first 5-10 seconds (until I reach 20+ mph). BUt - if I accelerate slowly, keep rpm under 2000 - it may go very smoothly and gain speed OK, but if I floor it - it doesn't respond very well, gains speed slowly and is very rough (shaking a bit). Also, during the winter time (when it was cold) it was less pronounced than when its warm (now). And wet weather (rain) seems to aggravate the condition.
The only thing I didn't replace is O2 sensor. Should I replace it also before I go any further? Or does anyone have other suggestions?

Thanks in advance!