Mercury Mystique Engine Runs Rough

Car Starts And Runs But Has No Power Sounds Rough And Engine Won't Go Over 4k Rpms. The Only Warning, I Had That Something Was Wrong Was...

When Holding The Engine Rpm's Up, The Engine Pulses High To Low. At Idle The Engine Runs Smooth.

When Ever I Stop At A Light Or Stop Sign My Car Shakes Bad But When It Is In Park Or Neutral It Doesnt Shake What Could The Problem Be. I...