I would suspect the power supply may on the way out. A new one is around £40 or less, and easy to fit. Get one with more rather than fewer watts available.
You want seperate rails if your going for a new power supply.
That way the draw on one device does not impair another (if on seperarte rails)
The overall wattage will depend on what you want to draw through the machine. if you have just standard bits a 400-500w power supply is ample - you only need more if you have a decent modern graphics card.