Lately my SC400's been acting up. I get what sounds like a "thumping" noise from the exhaust (can also hear it in the engine bay). Also, when you get on the gas (when driving), it stutters ocassionally. So the first thing I did was change the spark plugs. Afterwards the problem persisted.

Some details about the car:
~90,000 miles
Hasn't been driven in a month (I'm thinking stale gas may be a culprit)

Any ideas on what may be going on, would be appreciated.

 

I have the same mileage and have not experienced your problems, even after my car has sat for more than a month, so I don't think the fix will be as easy as getting a new tank of gas.

Remember that changing the spark plugs does not a tune up make- the wires and/or distributor may be the culprit (I got new wires at 45k).