The rear stiffner/brace might be bent or too soft after a lot of use. It might allow too much chassis flex. Replace it or upgrade to aluminum one. The radiobox also stiffens the chassis a lot. The screws tend to become loose and unthread the holes of radiobox. Consider getting a new radiobox for the reason or put loooooonger screws in (you have probably done it already).
I check, clean, relube (Hudy Premium Grease) and sometimes replace the diff pinion bearings before every meeting. I tighten the 3 screws holding bearings completely and then loosen them only about 1/4 turns. I also put a lot of grease around the bearings and then few drops of motor oil, so there's a lot of lubrication going on
![Laughing :lol:](./images/smilies/icon_lol.gif)
So far 1 bearing failure during 45-min final but it lasted till the finish. Can't complain!
Hope you'll find what's causing the problems! But for the first thing stiffen up the chassis!