Halogen lights working intermittently?

I had three 12V 20W halogen downlights fitted in my kitchen probably about 6/7 years ago. Up until Sunday they'd all worked fine but now they resemble very slow disco lights - they stay on for 40 seconds, go off for 40 seconds etc. I think a power surge may have kicked this off but I don't know how to fix it. I've removed the bulbs one at a time - still the same. Put them back in a different sequence - still the same. HELP!