2000 LeSabre Runs Rough Highway Speeds
 

Have a 2000 LeSabre Custom with 65K miles. Owned vehicle since new. Highway cruising at a steady 55 MPH steady speed, then accelerate to 70MPH, the engine bucks a bit, but will smooth out after a short time. No check engine light, no pending codes, fuel economy has decreased about 5 MPG. Now the replacement list; Intake Manifold, TPS, Plugs, Wires, Coils, EGR valve, Oxygen Sensor, Fuel Filter, Air Filter, MAP sensor, and Catalytic Converter (it was bad - the elements were loose). Cleaned MAF, cleaned Injectors (while Intake was being replaced). Checked fuel pressure; idle 45 PSI, idle with regulator hose off, 55 PSI, accelerating to 3500 RPM, 55 PSI. Compression check; all cylinders between 200 and 210 PSI - compression builds quickly. Engine oil changed at 3K mile intervals (Mobil 1 10W30) and drinks 1/2 quart between changes. Any ideas? Wish the PCM/ECM would give me a hint. Perhaps ignition module? Knock sensor? Please help me save a few bucks here and get this old ride back in shape. Thank you.