Why are you paying $170 each for these?!?! If you are replacing them, look into the 6920... You can get new ones for $123 or refurb "A" grade for around $100, both include a 1 year warranty and they are 10 times the phone the 5304 is, but if you really like the 5304 sets, you can get grade "A" refurbished units for under $50, also with a 1 year warranty. These are ones I found very quickly just looking at pcliquidations.com, but metrolinedirect.com, atlasphones.com, and startechtel.com all have them around the same price.
The most likely issue here is the phones are just failing... they are a minimum of 5 years old, probably closer to double that, and are getting toward the end of life as Mitel is slowly migrating away from the 5300 series phones altogether. I would not put any real effort into them or get new ones. You obviously don't have a maintenance contract with your current vendor that includes phones, or they would be taking care of this, so they shouldn't have an issue with you going elsewhere for devices as long as you use them for service and major hardware.