At the risk of stating the obvious there is one helluva lot of vibration going on with this car!
When assembling anything -or fixing as is the case here - I strongly advise using Loctite 243 Lock and Seal( or similar).
This helps significantly to stop this happening.
And double joy, its easy to use (a tiny amount goes a very long way) and ultra low cost!
When I had my older bikes a tube of this was always to hand!
