If you're having problems with Bluetooth in iOS 8, you're not alone. A number of users are reporting Bluetooth connection bugs after updating to the firmware, or purchasing a new iPhone 6 or 6 Plus—particularly when trying to pair their handsets to their vehicles.
Looking at this lengthy thread on Apple.com, and this one in the MacRumors forums, the issues appear to be fairly widespread. Within the last two weeks, users have complained of hitches including not being able to pair their devices at all, and random disconnects.