CAT TH337C Transmission issue

Hello buddies, I have one issue with my TH337C. It has only 900hours, and when I go forward or back with the reversing lever. Its make a jolt, not everytime but 8/10 times. And thats shake too when I up my gear to 1->2 2->3 3->4. and when I decrease them too. I already check the 2 discharge valves on the shift box, to see if they are not full, or in bad state. Nothing happened (theses 2 in red box on my picture). But I called my sellers, and told me they already replace the box (that I mark in yellow) at 154hours. The one on my tool looks already worn, all rusty I already drain and refill my gearbox, Everything was ok. I wanted to know if you have some solutions about this problem. Thanks