Two other things to consider.

1) If you feel the need to inflate the tire over its recommended max, you will be better off with a wider tire.
2) It might be due to poor quality tubes that are failing when getting baked in the summer sun. If you start the morning, with an over pressure tire, it is possible that the temperature increase during the day is causing enough increase in pressure to rupture the tube (the slit you noticed)...

Personally, I like schwalbe tubes and velox rim tape... both of which I get from Amazon.com