I have Powercolor 6700XT Fighter, never have this problem. That mean i have good GPU? )))
I think have reason for your problem, and before start to find problem, you must to know alt-tab is reason or not.
alt-tab is switch 3d mode off and on, power consumption is high than normal for short time, frequency is high too in this moment. First check your PCU, enough power for videocard or not in peak power. Check cable, you can use HWinfo in sensor mode and log what happend in alt-tab moment, and check maximum power and frequency. You can use standart windows monitor health system for check what give this freeze, you need step by step move on your target. This is 90% is hardware problem, but i not be shore is only videocard problem, need check all system how it work.