Hi,
I've found exactly the same problem with a N95 8GB phone as ggm02 has described.
For the same reason as ggm02 I'm using an external bluetooth gps receiver. The receiver works perfectly with Nokia maps, google maps, etc - no loss of signal or hanging of applications. But Sports Tracker (or the tracking part) will hang after a few minutes / km, while the stopwatch keeps counting.
I've just done a very quick try of ggm02's solution (staying on one of the charts) and over a 25 minute jaunt at lunchtime it seems to have worked.
I'll try more fully later and let you know if this workaround works - let's face it, this is a workaround not a solution

Hopefully Nokia will sort this bug out in a future release.