MacBook Pro - Crashing with external monitor

Hi Folks,

Got a right headache on here let me try and explain... First off I’ve got a 17” Early 2011 MacBook Pro running High Sierra and everything ran fine until yesterday... The MacBook usually lives on a shelf and I use an external monitor (via mini HDMI port) a Magic Trackpad and keyboard.

So there I was doing stuff, the display flickers a couple of times and the MacBook Pro drops dead.

Then I tried resetting PRAM and SMC then booting in Safe mode and Recovery mode - nothing works, getting partially through boot sequence, get a white screen of death with safe and a blue screen of death with recovery. Tries internet recovery mode - it also gets part way and dies into a white screen of death and a couple of boot loops before dying. I walked away to curse and drink coffee -

Randomly tries to boot it normally again a bit later and bingo macbook lives...

After a couple of successful re-boots with MacBook in middle of desk surrounded with paperwork I dumbly decided this morning to try external monitor once more... Started to boot and quickly crashed once again.

Exactly the same again - tried SMC, PRAM, Safe, Recovery and Internet Recovery and nothing works, all of the boot options end freezing in a frozen screen of death. Left it for 10 and hey presto it boots...

Have seen threads online about display issues with the current version of High Sierra but they all seemed to be in February and no-one really has concluded what the problem is or how to remedy it...

Any help much appreciated what can I do / try to get external monitor back in use, and also any ideas what’s causing the boot issues? No idea which combination of everything works - but seems only to work after I’ve left it to sulk for 10 mins.

Many Thanks

Ralf