I'll do my best, I hate glitching problems personally myself. If you have a spare servo lying around, preferably an analog one, give that a try first. The key to solving any radio giltch problem is to be patient and change only one thing at a time. This process of elimination will help you pinpoint the problem. Start with the following:
1. Change the servo. If you still have the problem...
2. Change to a different set of crystals
3. Change to another receiver
4. change the module on your radio (if your radio has one)
5. change speedo
Since you didn't have a motor installed, you can eliminate that from your list (it's kinda strange that you would have that severe of a radio hit without a motor). But when you put in the motor, make sure you have the proper amount and the correct type of capacitor on the motor.
It is rare that a servo can cause radio problem, but it does happen. Good luck and remember to be patient!