Very Difficult To Diagnose BSODs At Random Intervals

I've had a computer running rock solid for about 18mths. I've just used ~70% of it's parts to build a new computer for my dad and since then he's been experiencing constant BSODs. The computer is running a freshly installed over-the-counter bought OEM copy of Windows 7 Home Premium x64 which is up to date as of last week. It will run fine for an unpredictable length of time between 15mins to half a day then BSOD. Sometimes it will then get stuck into a "crash-reboot" cycle and he will have to hard power it off.

The computer is less 18mths old. The parts I have been using confidently over the last 18mths are as follows:
- MOBO: Gigabyte P55-UD4
- CPU: Intel i5 750 quad core (never overclocked).
- RAM: 2x(DDR3 2GB G.Skill 1333mhz)

The new components we installed are:
- GFX: Asus ATI HD5550
- HDD Sata 200GB Baracuda
- CD Drive: LG DVD writer on an IDE ribbon
- PSU: Antec VP450P 450w

I initially had an old install of Windows 7 on a 120GB Sata drive but when he started getting issues I installed a brand new OEM Windows 7 on the Sata 200GB and even changed the Sata cable. It ran fine for a day then started up again. From what I could see none of the crashes pointed toward gfx drivers - however I've noticed CPUz will only tell me that it's a HD5500 series card and gives me no other info so maybe it isn't being recognised properly . The only other thing to change was the PSU - which admittedly is a budget one (but from reviews should be reasonable) from deduction it's the only thing I can put it down to.