[GLLUG] How to troubleshoot router/modem hardware defect

[gvim]

I have a Draytek Vigor 120 modem connected to a LinITX router running 
m0n0wall. All ran well for many years until about 4 months ago when a 
lot of packets started dropping. At first I thought it was my PlusNet/BT 
connection but after substituting my old cheap Chinese Tenda router for 
both everything has been fine so my hardware seems to be the problem. I 
don't know which of the 2 devices is faulty. Any ideas how to identify 
the defect?

gvim