if its never had a tune up it over due by 35,000 miles. so id start there.

the cat. you can remove the o2 sensor from the exhaust manifold off the back of the engine, (pre cat o2) doing so will cause a large exhaust leak but if it runs better the cat is clogging up.

you can also after a good spirited ride at night pull over and see if the cat is glowing red. if so its clogged.