startup of iMac slow, graphics slow, problem is intermittent

when I start my iMac, it sometimes just ends up in gray screen

other times it starts but process is very slow, once up, everything graphic runs very slow

when I click on the notifications sections, it has many multiple red flickers and is VERY slow

it then is often unable to shut down or switch to other user account

the sound up down volume does not work

made appointment with genius bar, he suggested I run startup diagnostics of hardware

did that, all showed perfect

HOWEVER, all of this went away and everything was fine. then would return, then go away. cannot seem to trace to any application or use. or to if room hot or cold or running too long (perhaps actual chip bad solder/seat on board?)

iMac 2.7GHz Core i5 21.5 screen 10.8.5

suggestion on next step to solve/fix this?

thank you