GSM/GPRS Crashing but Everything Else OK

[Guest NightFox]

I'm finding I tend to have to do a soft-rest on my MDAC III about once a day, because although the phone continues to function fine, the radio (i.e. the GPRS/phone, not the FM radio) crashes. All appears fine, the device still works and has a signal showing on the bar, but I won't get any incoming calls, I can't make outgoing ones, and when I look at Activesync, instead of saying "Exchange: Connected", it says "Exchange: Last synchronised at 10:48am", which I guess is when the thing crashed. It's very frustrating because the only way I realise this has happened is when I notice that I haven't had any e-mails for a while.

One thing I have noticed is, working in London, I use the tube, and on my normal tube journey the train goes above and below ground, so the signal frequently comes and goes. The times it shows me I lost my ActiveSync Exchange connection usually corresponds to these times, so I think it seems to be that the phone loses the signal, but then never picks it up again (even though the top bar shows it's got a signal).

I'm currently using a WM6 ROM (3.7), but I also had this with an earlier WM6 ROM and the original WM5 ROM.

Anyone else have problems like this?