SL320 won't go above 40-mph...HELP!!

[carloz]

I own a 1995 SL320 V6 with about 180,xxx. The other day I was driving around 30-35mph, when I started to go above 40mph, it started to act like it was choking. If I try to push it beyond 40mph, it acts like its dying, but slowly it goes to 42-44mph.

I replaced the fuel filter and air filters, no resolution yet. What can cause this weird problem and how can I properly diagnose it?

thanks!