mechanic says i need a 1,200.00 valve job. before i took the car to him it ran fine. just a light skip. now it shakes and rattles when i let off the pedal. give it gas and it smoothes out. how can i tell if i need a valve job or new plug wires. it's difficult to reach the left side plugs. he put cheap plug wires on twice is there also an easy way to check the plug wire to see if their good ?. thanks for any info.