Why does my car shake when accelerating? on 2005 Subaru Outback

I feel my car shake through the gas pedal when accelerating. When I let go it stops. It has new tires & brake pads. The check engine light is not on. I changed the spark plugs about 20K miles ago and used iridium plugs car has 108K now (idled rougher with the iridium plug but never shook). No obvious lack of power and is smooth when you start it. A friend said it was the rotors but, I really don't know. Thanks in advance.

by in Los Fresnos, TX on February 02, 2012
2 answers 2 comments
ANSWER by on February 02, 2012
PUT YOUR REGULER PLUGS BACK IN .SOME NEW ONES YOU SHOULD STICK WITH FACTORY PLUGS MAKE SHURE THOSE PLUGS ARE NOT FOULING OUT CHECK THE BREATH AIR FILTER.MAKE SHURE THE PLUGS WAS GAP CORRECTLY TO .CHECK THOSE ROTORS THE THAY COULD BE WORE DOWN BAD IF THAY STILL HAVE LIFE IN THEM TO SHOP LET ORALIYS AUTO PART AND HAVE THE MIC AND TURNED IF NEEDED
COMMENT by on February 03, 2012
Thanks for the info. I'll take a look at those items.
ANSWER by on February 03, 2012
You need to determine if the "shake" is engine or brake/tire/suspension related. I have had good luck with Iridnium plugs in that engine, could a plug wire have been damaged? Make sure no oil is down the plug tube wells (if so change valve cover gaskets). It is important on Subaru engines if you have a misfire code set to check valve clearances I have seen them need adjustment at 100,000 miles. After that you are looking at brakes/steering/suspension.
COMMENT by on February 03, 2012
Thanks for the reply. I'll go back and check those things you mentioned.