I had the same problem w/ my RS. Replaced the thermo, replaced the radiator, did a flush and fill. No matter what I did, it kept coming back to haunt me. When I put a system in my car it got worse. Every time the bass hit it drove the heat up a tad. Turns out I had a bad alternator(seriously!). The thermo wouldn't open, making it overheat in idle(and causing the hose to rupture). Not saying that's what's wrong w/ urs, but it happened to me...

 

oh my bad i changed the thermo and its fixed the problem...the thermostat had grinding marks on it?? not sure what that was about but it hasnt done it since so im asumming its all better..