Imac keyboard malfunctions at startup
I would reset both your PRAM/NVRAM and your SMC. See below for instructions.

Reset your PRAM/NVRAM

Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). It may be worth you also resetting this. Here’s how to:


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again and you here the startup chime.
Let go of the keys and let your Mac reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. mouse speed, time and date/timezone, etc).

Reset the SMC

Your computer's System Management Controller (SMC) can also be reset. The instructions for doing this on an iMac are:


Shut down your computer
Unplug it from the power
Press and hold the power button for 5 seconds
Plug it back in and turn it on.