engine was chugging; got worse and engine light came on; mechanic said computer said coil 1 misfiring, replaced coil 1 and 6 spark plug (expensive plugs); still chugged a little; chugging getting progressively worse; tried gas treatment, and then seafoam too, but no effect; no engine light but chugging getting worse and now happening almost all the time; mechanic is stymied cuz computer reads no problems.

She has brought to the dealership and number of times and they have put on new tires and rotation, but nothing seems to fix the problem Any ideas???? Thank you