One of the likely causes of tiring in a game is the age of the squad. Quite a few of our regulars are over 30 (Murihead, Bannigan, Graham, Lawless, Robinson, Alston)
When you compare that with 2012/13 which was probably the last time we played a fast game that ran the opposition off the park then you can see we are more reliant on older personel.
Also worth noting that a team like Raith have very few over 30's which perhaps explains why they are still fresh enough to win games in the last 15 minutes. Similarly Dundee United have ran us off the park in both games and until they signed Wotherspoon they were only regularly playing 3 or 4 over 30's.
It would be interesting to see how our over 30 contingent compares to other Championship teams. To my mind we are more reliant that most and that potentially means fitness late in games can be a problem but more worryingly we will need to replace the core of our squad in the not too distant future.