Shudder during acceleration

[Craig Goss]

My recently acquired 1995 S6 shudders and loses power under heavy
acceleration (ie hill climb or aggressive passing).  By this I mean that
there is a rapid (several times a second) vibration that usually disappears
after a few seconds have passed.  The car does gain speed when this occurs,
but not as quickly as I would expect.  There's no boost gauge on this car so
no idea how I'm doing here, but performance doesn't measure up to that of my
'92 S4.

The car is going in for a check up on Friday.  In the meantime does anyone
have any ideas on what might be causing these problems?  Two of the ignition
coils were replaced a short time ago.  Any chance another might be
misbehaving?

Thanks!