The car runs great at speed and when accelerating. However, at idle, it shakes slightly. I just replaced the upper and front-right mounts, but at idle (700 rpm) it shakes. Any ideas?

 

RPM's is a little low. Usually a shake is missing. Have you scanned for codes? How old are the wires?

 

Behaves like a vacuum leak.

 

Thanks guys. I'll check both. But the CEL is not on.

The wires are probably very old. The PO was Nancy Neglectful. The heater core was SO bad the carpets were like a swamp, and the windshield IMMEDIATELY fogged when the defroster was on - it was more like a mister! She neglected the driver's door latch that it failed in a closed position - a delight to remedy! And I could go on.