Q: Grey screen on boot then shuts off

I have an iMac 27 Mid 2012 running whatever the latest update was 2 weeks ago.. When I turn the machine on I get boot sound and grey screen.. lasts about 8-9 seconds then powers down.

I have had it to the Apple store before and we replaced the ram and hard drive. It has been about 3 months and now this..

I bought this machine because I was told they were work horses.. I switched to Mac to avoid these issues .. GRRRRRRR

Suggestions.. ??????

David