Cyclic Redundancy Check (CRC) Error indicates when data is corrupted which can be caused by any number of factors.
MediaTeK based modems suffer from High U1 U2 and D1 D2 frequency issues and by reducing frequencies, this can help reduce CRC and FEC etc.
But, this can also be achieved in part by limiting synch rate, which inturn ensure cab/modem does not run at full synch rate.
Another option is to force SNR to a higher value as SNR is Signal to Noise Ratio – in other words a measurement in decibels of the Signal strength to the level of Noise on the line.
QoS to a degree can reduce FEC and CRC by not allowing full data transmission which can overcome errors/issues/congestion on the line/cab etc.
Trail and error I am afraid 
What would you suggest ?
In a completely sane world, madness is the only freedom (J.G.Ballard).
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