[Celt03]

Actually - I have seen this very thing happen twice. Both vehicles were new - both went in & out of the dealership's shops a number of times. The first one - the problem was NOT discovered by the dealerships but by an acquantaince of the owner.
The 2nd one - was discovered by my hubby because he had witnessed the first one.
Both were caused by a manufacturing defect in a plug wire that caused it to not seat correctly & slowly work thier way loose. The rough idle & misfiring would then ruin the plugs - resulting in the vehicle running worse & worse.
One vehicle was a Ford & the other was a Chevy.

And the check engine light never came on with either of them.
Put new sets of plug wires with new plugs - problem was solved..