Purple screen indicates a problem with the green signal in VGAs
By process of elimination:
1: Verify vga cable pin look good, not bent, and securely fastened to monitor and PC
2: Swap monitor (if possible), see if problem tracks monitor.
3: Swap vga cable, see if problem tracks cable.
If 1-3 don't isolate the problem then the issue may be in the PC, try swapping video card if you have a spare one, or check out cable and monitor on a friends PC. If it's the PC then all you can try is swapping the video card.