XRAY - Model racing cars

We have a problem whit the speed 300 motors (graupner).

The previous batches used to last for about 20 runs of 8 minutes.
The last batch we bought lasts only 3 to 4 runs. After that al the power is gone.
We tried this whit 4 motor. Al had the same life span.

The only difference I fount was the marking on the can.

The first batch was marked: Made in china Mabuchi 05 (green text)
The second batch was market: RK 370SD_5028 TD096Z19 (grey text)

Someone knows this problem?

Is there a solution for this?

Greetz
Bertracing

these are not the same motors, different number of winds

no same number of winds

I took them apart.