First thing I would do is replace the Bay Power Supply in the control bay, and make sure that cabinet is on a UPS, or at the very least a new surge protector.
Also, start getting printouts of the programming... Get a USB Serial adapter (or a second one) and connect it to the printer port and dump all the programming you can get. Not saying it's failing, but lets be honest, this thing is pushing 20 years old and it WILL fail eventually.
Most digital systems I tell people have a 7-10 year lifespan... Digital "PBX" systems like Mitel SX200EL, SX-2000, Meridian 1, etc. I say have a typical lifespan of 12-16 years. The customer should at least start planning for a replacement or upgrade path.