As you can see by the wide and varied amount of answers without you being a little more specific its a hard question to answer.
The GTSt has a tendency to miss just as peak boost comes in at about 4-5000rpm especially at full throttle. In can be a problem associated with the high KV (firing voltage) at this point.
There is a number of reasons and a number of cures for this fault, the more info you provide the closer we can get to an answer.