3-bulb fixture eats bulbs in two sockets

Hi, everyone; I have a 3-light fixture on the ceiling in my kitchen, and it seems that two of the bulbs seem to burn out far faster than the third. I originally had a 3-light track light that did this, both with 60W incandescents and with 13W CFLs. When we renovated last summer, we replaced the fixture with a new 3-light fixture using 50W G10 Capsylite bulbs, and it has done the same thing. One bulb is still going strong, but the one at the other end burned out after about three months, and the one in the middle after about another three months. The fixture is usually on for 3 to 5 hours a night, most nights. I've had the usual nebulous "wiring problems" answer from a couple of alleged professionals, but I would expect an issue with the house supply to affect the whole fixture equally, not cause the bulbs to burn out sequentially. There is only an unfinished attic above, and no doors within about fifteen feet to cause vibration issues from being slammed. Any ideas?