Not sure if you are still having the problem or if you got it resolved, but I had the identical problem you described and all you need to do, is to pressure check the cylinders. You might have a number of bent valve stems.

Reason: Down shifting, and accidently going above red line will blow your valves. The S2K are known for this problem and only $2500 later after a valve job my mis-firings disappeared.

If it was something else you are probably really lucky.