CG3000DCR - change config to block certain downstream channels?

I have a weird issue - I'm on a 25/8 plan, and normally get a steady 30/8. But about 25% of the time my performance crashes down to 1 or 2 Mbps if I'm lucky (sometimes 400Kb/sec or so!). After two site visits not really resolving the issue (including a swap out of an SMC modem), I think I've finally worked it out - there's interference on my cable around 600Mhz, and whenever the modem selects the 603 Mhz downstream channel, it gets a SNR of around 25 and my performance goes into the crapper. If the 603Mhz channel doesn't get a lock, my performance is just fine. I've been watching this over the last few days and it's absolutely consistent. The 597 MHz and 609 Mhz channel also suffer, but their SNRs stay around 33-35. All my other downstream channels get a consistent SNR of 42 dBmV.

When I get locked onto the 603Mhz channel, I have to reboot the modem many times until the luck of the draw removes it from the mix. It's super frustrating.

I called into tech support today, but the lady was unable to work out how to prevent my modem from picking up the 603 Mhz channel. I've got a tech coming out on Thursday, but I thought I'd ask here if anyone else has either seen something similar, or knows if the config file can be edited to selectively ignore certain downstream channels.