Sounds like a connection issue to me. I would clean all of the connectors and try again, also that laptop has poor heat distribution as I believe the single fan kicks on as the result of the CPU's temp not the GPU's. What you can do to help the heat issues is to get a copper plate about 1cm x 1cm and about 1mm thick. Take off the thermal pad on the gpu and apply some arctic silver to the gpu then place the plate on top then put some on top of the plate and put the heat sink back on. Make sure not to let the plate slide.
If you start getting artifacts then there is something physically wrong with the graphics card, you can try reflowing at that point using your oven if you dont have a table.