1.) Clean everything. Check with the manufacturer on recommended chemicals-- glass and window cleaners will often remove the non-reflective coating on many
c) reflector (if it won't come clean, replace it)
2.) Check the condition of the socket. If there's evidence of a carbon buildup in the sockets, replace it now, or at least tag it as a unit to watch.
3.) Check/replace the strain relief on the lead at both the instrument and connector.
4.) Check the wiring in the connector.
5.) Check the lamp; make sure it's the appropriate wattage. Virtually every company/theatre I've worked for has overlamped their x9s and x12s. Nothing causes more maintenance needs than an FEL where a EHG should be. :)
6.) Check the operation of the shutters. Replace if they're too badly warped or even burned through (Make sure that they produce a clean STRAIGHT line)
7.) Check the operation of the barrel. If it's finicky or sticky, pull it out and thoroughly clean out the tracks.
8.) Check the tilt-lock knob on the side of the unit. Does it lock?
9.) Tighten every nut/bolt/screw on the unit. Expansion and contraction of metal tends to loosen them.
10.) Check the C-clamp. Make sure it's not bent out of shape from being overtightened. Make sure the pan adjust nut still tightens.
11.) Bench Focus. Peak or flat, your choice.