Do you mean DMT rather than DME?
No, I don't think you can set a max sync speed. You could, however, increase the SNR margin a bit and then reboot the router in the morning. It would then sync at a higher SNR i.e. lower speed, with enough headroom so that in the evening when SNR reduces, it doesn't go low enough to force a resync.
That's pretty much the only way round it I can think at the moment. If your SNR is 15dB and you're still dropping out, that's some big fluctuations so reducing the SNR margin would make matters worse.