so..recently i went to Precision Tune to have the plugs and wires on my 2000 Montana changed at a cost of $240. After 3.5 hours of waiting, i get my van back and after about 2 weeks of driving i find that i have gone from about 17 - 18 miles per hour around town to approximately 14-15 miles per hour....what could be the cause of this problem? 
