I know its a cop-out...but I think the guys are very tired. It has been a brutal first 2 months, and it all seemed to catch up with them on this road trip. They just haven't had that extra gear in the second half of games that they had at the beginning of the season. I think part of that is because the bench has really not been able to maintain leads, which puts a lot more pressure on the starters, but more just because they have played the most brutal schedule I have seen in a long time. The built up fatigue combined with the road trip, and the holidays (remember they still have families, and have to deal with them over the holidays just like the rest of us) just seemed to form the perfect storm.
They definitely need to get some reinforcements to the bench, but I think the problem of execution late in games will solve itself as the schedule slows down in the new year, and they are able to get a little more rest.