Jeff had a great suggestion. NEW RIM TAPE. I had 3 blow outs last week on my rear tire. Here is my analysis and conclusion.
First blow out - was a brand new tube, had stan's in the tube, 120 psi and a backpack. It was a monday and my pack is loaded with clothes, towel, and stuff for the week. 5:20 in the morning, threw the tube away, it was cold.
Second blow out - riding home, blow out after I hit a bump. kept the tube and wanted to look at it.
Third blow out - pumping up my tire at night before the next morning - 5 minutes later - blow out!
Observed both tubes, both split, same area. Conclusion and visual observation - at the end of the rim tape there was a slight exposure of tube to rim/spoke hole. New rim tape $6.50 and no blow outs. Savings: no headaches of tube change at 5:30 in the morning, stan's $3.50 per tube, and new tube $6.50. Try changing your rim tape!