Ps2 problem, sound but no picture, HELP!

Hi peeps, any advice would be appreciated, when i switch my ps2 on i can hear all the sound but no picture is coming through on my tv. I have a swap magic flip top casing installed if that makes any difference? My playstation is about 4- 5 years old, not sure of the model number but can check if that needed. So exactly whats happening i don’t know, i have had it connected up the same way for over a year now (white, yellow, red plug stuck in the side of my tv and the picture coming out through av3 on my tv) but when i switch my dvd player on (which is ran through av2) it takes over the av3 channel which my playstation is supposed to come through and the sound of the playstation still comes through over the sound of the dvd so i’m baffled. I gave it a good clean when then 1st happened and it seemed to rectify the situation for 2 days but it has gone back to being knackered so if anyone could give me any pointers on how to get this sorted i would be most appreciative.
Thanks again
burga :o)