iMac 27" random shutdowns due to LCD screen?

Hi all iMac hardware experts,

my iMac 27" mid 2010 has been suffering from random shutdowns and sometimes wont start up again. When eventually it did start I checked all the usual suspects, temperatures, HDDs, fans but nothing explained the shutdowns. Monitored with TG pro and the logs didn't indicate any problems. I changed the PSU board and made no difference, I left the LCD screen out (I was tired of the constant in and out) and used the external display port with a Dell 27" monitor.

Ever since I did that it hasn't shut down once!

My order of likelihood:

1. could the LCD be cause the random shutdowns?
2. could it be the GPU because of the different port used?
3. LCD inverter board
4. Logic board?

any ideas?

regards

Harry