Trackspot lamp socket burnout

So I have a few fixtures that have started to burn through lamp sockets. Only seems to be happening to 3 out of the 16 i'm responsible for (1 of mine, and 2 of my employer's). The lamp socket contacts burned out. Replaced the contacts, a month or so ago, and now those are starting to burn out. A friend suggested that the heat filter might be going bad, but i'm not sure that makes sense, since the socket is on the hot side of the heat filter.

Any idea what might be causing damage to the lamp sockets?

thanks,

Jeff
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