Hi,
I had the same problem with this at my mothers place. Drop out, not connecting, conceiting then not etc. on the 2.4 MHz's went on for weeks. Did firmware update etc. Solution which has fixed the problem is to make the 2.4 only 20 MHz in the modem settings under advance. Problem has gone away, connects first time every time and so far no drop outs.
I hope this helps some people as I was pulling my hair out and of course Spark were saying there is nothing wrong.
Cheers.