The Engine Can Be Cold Or Hot It Has A Hiccup Every 2-3 Seconds. If I Have It In Park And Hit The Gas Up To 3000 Rpm For 5 Seconds And Release It The Shake Goes Away Temporarily, Five Minutes Later The Hiccup Starts Again. Now If I Hit The Gas Up To 3000 Rpm To Make It Not Shake, Then I Put It In Drive Or Reverse The Shake Starts Right Away.the First Zero To Ten Mph Acceleration I Feel A Hesitation After That It Is Smooth. I Never Had This Problem Just One Day It Started To Shake. I Have No Check Engine Light On. I Have Replaced The Plugs, Wires, Cap Rotor After The Problem Started With No Results. -also- Between The Shakes At Park I Can See The Rpm Gauge Go Up Slight When It Stops Shaking Then When The Rpm Goes Down It Starts To Shake. Any Thoughts? (1991 Sel 420)