Solved: PC freezes, bootmgr is missing error

I've been getting random PC freezes where the screen is frozen, I can move the mouse but can't click anywhere or CTRL/ALT/DEL out of it. It usually happens while playing games or watching videos but has occasionally happened while just navigating my folders.

When I reset after the freeze I usually get the following error in dos "bootmgr is missing, press ctrl, alt and del to restart". I can usually get around this by going into the BIOS and selecting the SSD that is my boot drive.

I've checked the cabling of the SSD and it seems ok as far as everything plugs in goes.

Any ideas on what to do here?

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 8.1 Pro, 64 bit
Processor: Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 4
RAM: 8159 Mb
Graphics Card: AMD Radeon HD 6900 Series, -2048 Mb
Hard Drives: C: Total - 122101 MB, Free - 46875 MB; D: Total - 2861458 MB, Free - 2751410 MB; E: Total - 99 MB, Free - 77 MB; F: Total - 953765 MB, Free - 701867 MB;
Motherboard: ASUSTeK Computer INC., P8Z68-V
Antivirus: Windows Defender, Disabled