Or the power draw is too much for the board to handle. Could try ditching some of the ram, dim screen and remove or turn off peripherals. As for baking; reflowed 7970Ms, 8970Ms and a 260M all with good results. Only thing is that I'm using a temperature-accurate hot air station whereas household ovens are grossly inaccurate when measured; dozens of degrees C above and below the preset value (cookies care less, presumably). The reflow windows starts at 217°C, so set oven to something like 200°C, wait for it to 'ping' and try the card. Failing that, add another 5°C, retry and so on.