Are the bulbs really bad or just developing a bad contact with the socket. These Malibu things are pretty much junk if it is the same stuff they sell at Home Depot.
What is the actual voltage?
Have to agree here, I have these at my house. If it is not a bad connection with the bulb contacts, it can also be the wire clamp connection, which is usually buried under ground. The wire clamp is also plastic and a pain to crimp tightly.