Faulty driver side Xenon Headlight?

I have a Ford Mondeo ST220 (06) which also has the upgraded Xenon Headlights. ABout a month ago the drivers side headlight (Xenon Part) stopped working, replaced the bulb and no joy. Googled it and found out that the Ballast is usually at fault so got a second hand one and replaced it and worked a treat for a couple of days then stopped working again. As I bought the replacement ballast second hand I assumed it was bad luck so bought another, again worked fine for a couple of days and stopped working again (AHHH)

I'm at that point now where I am considering buying a brand new part (£170) but concerned that that too may break due to an underlying issue?

The drivers side xenon (Broken one) gets it's feed from the passanger side (Working fine) so can't see there being an issue with that and the fact it works to start with is a little bit more confusing.

Can anyone point me in the right direction of there is anything else I should be checking before spunking a load of cash on a new part which may also break!

I would really appreciate any advice.

Many Thanks

Gav