All was fine until 3 of the bulbs blew. Got replacement bulbs and now they keep dimming. They'll be on nice a bright for about 20 mins and then dim for about 10 mins.
You said in your original post "a bit like this" and provided a link to a mains voltage light. Are your lamps mains or low voltage? If they're low voltage, could be a problem with the transformer driving them. If mains, I'll go with others, and suggest you get a professional in, sounds dodgy to me.
All I know...got it from B&Q to replace one single central light. A sparky fitted it less than a year ago and it has those little spot light type bulbs with two pins on the end. That's the extent of my knowledge. OH is asleep on the sofa...otherwise I'd ask him (not sure he'd know though...he only knows about tarmac..pmsl)
If mains voltage, it is a possibility that the dimmer is being over-loaded by too highly rated (wattage) bulbs. Replace bulbs or fit upgraded/different dimmer.
If low-voltage .. check transformer and bulb ratings. Low voltage transformers with several bulbs do not take kindly to higher rating bulbs that take transformer over the rating threshhold.