1999 clk 430 wont go over 20ish mph?

1999 clk430 150,000 well maintained. A few mornings ago when I tried to start my car it started then turned off two different times before it finally stayed on. it did that again later that afternoon when I left class. Then,that evening I was driving and out of nowhere it just felt like the engine switched to a lower gear. I pulled over and put it in park,then took off again.Butshe wouldn't go above 25 or so. I'm not mechanically inclined, so its hard for me to describe this.Does anybody have an idea of what might be wrong? Approx $$?