Hi Simon,
I had a similar issue on my cart. I was running (max at one time) 7 411's: 6 lavs and 1 wireless boom. The boom channel kept dropping exactly as you describe. It was only the boom channel though. All along, I thought it was the mixing console (Mackie 1604 vlz pro) I tried new cables (though they're all pretty new to begin with) different channels, everything I could think of... and even retired said mixer as a result. It was an old board, lots of hours, high current draw -- not really designed for location cart mixing. I thought for sure it was something in the board. So I got a CS208D : ) (I was planning on upgrading anyways)
It's good (in a way) to hear that someone else has experienced this phenomenon though. I thought I was going crazy! I still haven't figured out the cause, but I think it had something to do with some kind of resistance build-up (for lack of a better term) in the signal path. I say this because no amount of fader or gain adjustment would remedy the problem. What DID work however was simply reaching around the back of the cart and disconnecting / reconnecting the line. The signal strength would return to normal just as you described when you'd change the output level on the Lectro. It got to the point where I'd have to do this 5 or 6 times a day (usually after lunch.) So I could see why it would kind of seem like a heat thing.
In any case, the common denominator seems to be the 411's. That said, I've used the same 411's with a 442 in the bag (along with a 702T) all powered by a Remote Audio BDS/ IDX LiIon NP1 set up -- running all day (10-12 hrs) without a hitch. The signal drop only occured on the cart with the Mackie.
So, I'm not sure it's a heat thing because my units all work fine in the bag -- 4 of 'em, all Velcro'ed to each other including a Lectro IFB trans, the 442 and 702T -- all generating heat with little to no dissipation. On the cart, the Lectros were basically in the open air -- nothing but the casing to trap heat. This is what lead me to think it has something to do with some kind of resistance build-up in the output signal. The only question I would have at this point is how old your gear is.Â
Having learned about your situation, I'm inclined to CC this post to the folks at Lectrosonics and see if they have anything to say.
I'm sorry I don't have a remedy for you, but perhaps this info will help? Please feel free to eMail me:
tom@taylormadeaudio.com