You do not say what type of system you have. On some systems, if you have any metal to metal contacts that can rub together under vibration, it can create some interference. It is not as likely to happen if you are on 2.4GHZ but will often happen on the lower frequency such as 72MHz, 6 meteres, etc. Of course, if there are any intermittent connections any where, vibrations from the engine could cause such problems.