Summits consider overdrive the same as overtime and is strictly voluntary. Either you are driving this poor vehicle too much or you have offended it somehow. Excessive driving is easy to solve. Try giving it some structured time off. Offences on the other hand can be very tricky. You may have to try several things (new tires are a popular solution).
It sounds like you are having transmission problems. If the transmission seems to be working ok you can check the fuse for the overdrive to make sure it is still good to even put the power on to the overdrive.
Your over drive might have gone out, you will need to have it checked out to make sure. A repair place can tell you if it is going to die, and maybe fix it, or you may need a new one.
