Connection problem between Toshiba laptop SATELITE...

Question

Hello, I hope someone can help me!
My problem is that I cant connect the two devices via a VGA cable. Weird is that when i use an S-video connection everything works fine but when I use a VGA cable the signal doesnt stay stable for longer than 5 mintutes. It goes to a search mode looking for the right signal as if it doesnt recognise the connection. I ve used various types of resolution on the laptop (from 1280x800 to 800x600) to match to the projector's original(800x600) but it didnt work. i tried to connect it with a siemens amilo laptot same problem...any ideas why this happens? and why the S-video works fine instead?
Thanks a lot in advance, maria