Astro boy.

It is nothing to worry about.
My 1200GX also did it.
Especially on very hot days.
My dad told me a trick that I use on all cars that I drive and it has always worked.
Hot or cold weather.
Put the car in second gear and then select reverse when you want to reverse.
It never failed me.
I do it with my datsun.
Can select reverse quickly with no problem.
Never select reverse when your car is moving forward or backward.Even when moving very slowly.