2001 Buick Century - Stumbles on Acceleration

Hello,

My car is a 2001 Buick Century 3.1L V6 with approx 85,000 miles. Six months ago I had the wires and plugs replaced by a local shop. About a month after the engine started shaking hard and the check engine light came on.

The dealer said the rear wire(s) had burned on the exhaust manifold. They said it would need new wires. I mentioned they had recently been replaced and they suggested I go back to that shop as it should still be under warranty than pay the $200 for them to replace the set.

Long story short the first shop reluctantly replaced one wire and said that since the dealer reset the check engine light they could not diagnose the problem any further or determine if they fixed the problem.

Four months later my car seems to vibrate more at all speeds and stumbles on acceleration. My gas mileage is 3 to 4 miles per gallon less that it had been before the tune-up.

HELP! What should I do? Go to the dealer and have the professionals replace the wire set?

Will I need new spark plugs if the bad wires caused the cylinder not to fire?

Thank you for reading my long post and I appreciate any response. :D