27” i7 mid 2010 iMac - control device problem

Hi,

the last week or so my iMac has been acting really strange!

after switching on and getting to the login screen, everything is fine. However as soon as I enter my login details I can hear a very fast & repeated duh, duh, duh... sound, almost as if a key is stuck or I’m trying to cluck on an invalid command.

everything however loads from the login screen but no matter what app or program I select, nothing loads. Also if I use the trackpad, or mouse... I can’t select any options as the drop down menus disappear as soon as they are highlighted.

the only resolution is to reboot.

so far after 2 or 3 reboots everything is fine again.

ive tried a couple of keyboards & trackpad & mouse and the issue is still there upon the first startup of the day.

any pointers would be greatly appreciated.

Cheers