Monitor comes on, but then dims (still visible)

Hi, I have 2 LG LCD monitors (LG FLatron L1510S) both with the same fault. When you switch them on, they come on for a second and then go dim (the screen is still vaguely visible), any ideas?

I had the same problem with some Acer screens which ended up being capacitors, they were obviously damaged (bulging heads) but all the caps in these monitors look OK.

Any help greatly appreciated.

Thanks