CHASIS INTRUDER! SYSTEM HALT error message

Mobo P5SD2-VM Asus –got chasis intruder error on staring comp. On a restarting error goes away. Only to return on starting after sometime. I have used my comp. for 3.5 years without any problem.
There is a plastic cap over chassis signal and gnd. soln. tried – cleaned Mobo. — took the battery out and placed it back. — could not find a setting in BIOS for chasis intrusion. — placed RAM in another panel. Please help. Is it pointing to a bigger problem with my Mobo.?
