Auto Gear problems?

I have 416 Sprinter 2.7 TDI with an auto gear box that works fine and from time to time just goes into manual on its own. I then have to gear shift by hand or stop ,turn off and restart before it will operate in auto again. In manual it does not have full power. It had a aircon fitted with a radiator fitted in front of the motors one and an elec fan in front of that. I have found that on hot days up hills the motor heat gauge moves up to 100 and only then does the fan come on.
Could it be over heating ??. By using the heater and the aircon at the same time does help to keep the temp down.
I also found that the brake switch at the top of the brake peddle arm was not getting pushed in much when the brake peddle was released and the brake lights were staying on. This meant that the contacts in the switch were burnt and it also effected the gear change. I replaced the switch and for a time my problem seamed to be fixed. I though of fitting a heat sencor that will bring the fan on sooner ??. Can you help.