Ive got a Bush 22" tv that has same issues with my ps2 two. The problem lies with pin 7 (AV Switch signal). The tv is using whatever voltage is on this pin to detemin if the screen should be 4:3 or 16:9. The system needs to provide at least 9.5v-12v to stay up in 4:3 mode, else anything between 2 and 9.5 is 16:9. 0-2v and it wont switch.
The kicker for me is that the tv isnt even a wide screen yet follows the above rules. The output of the switch signal appears to me 9v so my tv doesnt hold 4:3 image very long if at all :/. Also my tv seems to require the blanking signal else the screen stays blank on rgb. The ps2 rgb cable connect pins 7 and 16 together (blanking) with a resister bringing the blanking voltage to the required 3v. (Cable actually didnt have resister, just connected two pins - blank screen on my tv).
My Current solution is to use a scart switcher box and switch the dvd player on at same time (Interestingly needs the resistor too else blank). this puts enough voltage on the switch signal to keep it 4:3.
My Amiga has no issues with rgb , its cable provides 12v on switch signal and already had resisters on Blanking and composite signal (Both are needed to work on my tv). I think Bush must be super strict on the scart standard or something because it requires abit of work getting things working on it.