Counter-Strike: Condition Zero

this problem always happens after 10-15min of playing the game:

When I press the move keys there is a 'beep' sound. And the beeping continues if I hold down the keys.

When I try to press the console tab down, I am unable to type anything. And '?????????' appears in the typing field.

I am also unable to buy items using the number keys.

Has anyone encountered similar problems like this before? Please help :(
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Even a broken clock is accurate twice a day