Memory, video and overclocking errors

Self built machine in 2003 worked great until recently. It’s a P4, 3.0 ghz, Asus P4C800 Deluxe board with ATI 9600 video card. Memmory is 2 x 512 DDR400. All drivers up to date. Bios not flashed since a year ago. When I start up the monitor does not load and I receive an over clocking error (F1-F2 choice). I never overclocked and checked the BIOS for settings. I shut down machine and start up again. This time all works fine except only 1/2 the memory is registering. This happens 3 out of 4 start ups. Reseated memmory with same results. Memory is in correct sockets A1 – B1.