How can I fix constantly resetting HD 4000 GPU?
If your issue appeared with 10.10.3, I am 98% certain it is a bug in OS X Yosemite, introduced with the 10.10.3 update and related to Intel HD 4000 GPUs. Welcome aboard. The kernel panics are totally erratic, some users have also graphics glitches like you describe. The only common feature is having an Intel HD 4000 GPU.

However, your case seems more severe than that and could also be caused by a dying GPU. 

On MacBook Pros with dual GPU you can avoid the problem by disabling Automatic Graphics Switching (System Preferences > Energy Saver). 

Please contribute to the thread in the apple discussion forum and file a bug report (with link to the thread).