It may be that the mechanism of the keyboard is being affected by the hot weather. Remember there are more than 100 key tops plus their associated moving springs, levers, thingies and gizmos which can easily be affected by expansion.
If you have a USB keyboard, un-hitch it and put it in the fridge for a while - not too long mind, you don't want condensation on it. Re-attach and see what happens. Be careful doing this if it's a PS2 keyboard - power down first before you unplug.