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.
The ST585v6 is an excellent router, but the external shell design is not the best. As you say the vents are on the wrong side. The curved sides are a nuisance and they used to be sold with a little plastic cradle which allowed you to stand it up but for whatever reason (cost probably) Thomson decided to not supply it (I didn't get one with mine). I use a couple of big blobs of good old blu-tack to stand it up on end. At least the shell of the v7 is now square and can be stood on end, although it unfortunately doesn't have the same chipset as the v6.
Although some people may find this a bit extreme I annually open up my hardware and carefully remove any dust. I also if possible tend to stand up my equipment with the psu at the high point as I have found that this generally tends to be the warmest part of the device, and doing this helps to prevent thermally stressing other internal components.
