I've run both the R603FF and R603FS, the FS is the one with two aerials, I couldn't tell the difference between them in range or resposne.
My understanding of the over heating issue is that it is a component tolerance, rather than an intentional thermal cut off. So basically they stop working, the actual temp varied quite a bit between receivers. I believe Futaba started testing them and basically not selling the ones with a low tolerance. A gold spot on the receiver should indicate that it is a more recent receiver and should be okay, even some with out gold spots should be okay. I don't believe the problem has ever been due to heat actually generated by the receiver. I think the heat can only come from either something hot near by, or the high power servos pulling current through the connectors and PCB. The receiver shouldn't be getting to 60C, I run them on electric cars and I've never noticed them above ambient. If your box isn't touching anything hot I would make sure you've tried bypassing the receiver with the red lead, not even connecting the servo red lead to the receiver.
Most of my understanding has come from talking to Ripmax the UK distributor, give them a call, they may be able to help, especially if you have one of the older un-tested receivers.
http://www.ripmax.com/rm_contact.asp