On older Source Four burner assemblies without the lamp retainer wire / clip, the lamp may work its way out of the socket as the fixture heats up and then cools down. This can cause arcing and pitting of the sockets and the lamp pins, which will result in damage to the socket. Eventually the socket and lamp will become corroded to the point where they must be replaced.
To avoid this:
Open up the endcap and verify the following.
If needed, replacement Source Four UL Socket Assemblies are ETC Part# 7060B7019.