Bowdenham particularly is laggy around Apsley town centre when you've got about 10 O530s going round near identical routes...
Sounds to me as if OMSI is searching for a non-existent AI Vehcile, as 0118 said, check your logfile. If you can't find anything in there, verify your game cache through Steam, there's a thread on here telling you how to do so, somewhere.