I have an iRoast 2 - some playfully call it the "iRoar" - which is made by Hearthware, Colin, so the machine you gave away should be quite close to this in tech specs (it certainly sounds similar in roasting smoke!).
I do not have any problems with the disconnect, but will offer more of a preventative solution. When storing the roaster, I sit the top part (glass) on the base and turn slightly, but do not engage the spring & lock mechanism. Using the spring only when roasting keeps it newish and strong. I wonder if replacing the spring on yours will help prevent the disconnections and early cooling? Would be a cheap & easy fix, if it works!
P.S. When mine dies - it is approaching 2 years old now - I have decides to go with a Behmor 1600, practically the same price point as iR2, but a drum roaster for home use. Apparently contains smoke well.