I have two problems on my iMac

1) When awakening from sleep, my iMac that is, the mouse and keyboard
have frozen.
The mouse is plugged into the keyboard USB port, which in turn is
plugged into a rear USB port on the iMac
The only way I can cure this is to power down using the rear on/off
Upon powering back up, the problem has cured.
This happens once or twice a day, sleep is around 5 times a day.
I have other devices plugged into other USB ports, but they are
usually off most of the time, ie printer.
Help please.

2) On powering up, just the past few days, the display is noticeably
If I select display preferences, then color, then close that window
without selecting or changing anything else, the display reverts back
to normal brightness
Help again please.