Volvo: Not shifting, brake light switch, overdrive switch

Question
Our 1998 Volvo is not shifting in the afternoon when it is hot. We need to use the overdrive switch. Our mechanic says the brake light switch is okay. He cannot find out what is wrong. Do you know what could be wrong? Thank you!

Answer
Chris,
A shifting problem is one that requires some hands on and road testing, along with the model of the car. Best advice find a better shop one that knows Volvo. As we work on the same car everyday we get pretty good at fault tracing.
A year model 98 will have OBD II diagnostics, there might be fault codes that will lead to the fault.