Mine had issues in the heat, or I should say,more prone to problems in the heat, it ended up being faulty/dry solder joints in the keypad module. One way that you can test this, next time its failing to start, tap firmly on the dash between your stereo and glove box, while trying to start the car. May not always be a tell tale, but it can be.

See if you can get the connections re-soldered and see if it helps your problem. (Subaru's diagnosis was to replace the unit, where as my bro re-soldered the connectors, now alls good)