Just a couple of things to mention.
Speedtouch routers, whilst good, can get very hot- The air vents are on the bottom I personally run a ST585v6 upside down, and I know several others that do similar, by propping it up etc so that it can increase air flow.
If its working hard (more traffic or dealing with more FECs) then theres a chance it can get warmer than normal.
Also with you saying its not occured when as many people have been around? Check the location, it it by anything electrical that could be being switched on that could occur to interfere with the signal.