Mazda :: 2004 - Idle Bounce While Coasting After Valve Cover Gasket Replacement

I've got a 2004 Mazda 6 with the 3.0L V6 & a 5-speed, with about 128k miles.

The car runs fine, but was leaking oil onto the exhaust manifold and so I recently had the valve cover gaskets replaced. the mechanic could not get replacement gaskets from Mazda, and so changed the valve covers themselves also, and consequently needed to replace a couple of hoses that connect to the valve covers.

Since the repair, the car drives fine - acceleration seems normal, I don't notice any performance issues. The CEL is NOT lit.

1) When the car is in neutral, but rolling, the idle consistently bounces back and forth precisely between 1250 & 1500 rpm. This occurs always, and happens whether the gearshift is in neutral or the clutch is depressed. With the radio off I can hear the engine surging when this is going on. As soon as the car comes to a complete stop, the idle immediately drops to the a rock-steady position just below 1000 rpm, as it always has.

2) I assume related to the above, but since the repair my gas mileage has dropped from its usual 18-19mpg to 15.7-15.9mpg as recorded at two fill-ups, and my current tankful is looking to return equally low mileage.

I brought the car back to the shop and the mechanic took the car out for a drive with me, with his computer hooked up while we drove. He found nothing wrong, nothing out of whack, and saw no evidence of any air leaks.

1) Seems strange to me that the valve cover gasket couldn't be obtained, and new valve covers were needed. I want to believe the garage is being straight with me. Is this really plausible?

2) What could cause the idle to behave this way? I understand what a valve cover does (in my earlier years I changed the cylinder head on my old Saturn) and can't figure out how or why changing the valve covers would have an effect like this.

That leads me to conclude this is more likely caused by the new, slightly different hoses attached to the valve cover, but I don't know exactly what those hoses do or why they connect to the valve covers, so I'm stuck here.