Replacement card could have been installed improperly. If you leave the laptop off for a while, does it work OK when you turn it on at first? Did the fan also get replaced or just the physical chip? Try running the laptop with the cover off to expose the chips to the air, make sure the fan works OK. Does it work fine with that setup?
Screen connections checked?
Thanks for suggestions - I cleaned the fan (not that there was much rubbish there) - just replaced the card - have not run with cover off although point well made - I did not actually see the fan working (although it span freely) - also downloaded latest drivers - fault seems intermittent and 'repair' seemed to be ok for first day - your thoughts?