TV Toshiba field sync problem during first 10 after power on

[Jordy]

I have Toshiba 2979UE, after turning on it is normal for about 2-3 minutes then the horizontal/vertical sync start going crazy for about 10 minutes before it comes back to normal.
I have tried to resolder almost all the solder joint, which does not make any different.
The problem usually goes away if I shook the PCB. could it be the hairline crack, but where ?. The funny thing is once it comes back to normal I cannot reproduce the problem by shaking it. Any idea ?? :