alvertal

I started to notice some shaking when transmission is in the 3rd gear or when switching from 3rd to 4th. It doesn't happen on flat road, but small bumps or slight road waves can make it happen. Sometimes 3rd gear stays little longer that I think it should, but I'm not sure if it is related to the shaking. Shaking feels like wheel hop, but at low speed and low torque.
I have checked transmission and motor mounts, I have new struts and bushings.
Is it possible that some solenoid is not working properly or I got some faulty pressure sesnor on teh transmission?
Any advice would be very helpfull. Thanks.