Since the other values are 0, then ....
You had a period with several loss of syncs through noise bursts Or...
RouterStats was unable to obtain any figures from your router, this could be a network issue.. Or it just sometimes happens that figures aren't graphed for some reason.
Did the downstream have corresponding dips at the same time. I would expect it to have. If it didn't then I'd more likely suspect a reporting issue with routerstats.
Are you graphing errors? If the error rate went up at about the same time then its most likely several loss of syncs.
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Sorry I know Eric had already answered, but I just wanted to expand a bit more....
If you are using routerstats you can sometimes record CRC errors as user defined.
Alternatively you may have a router which is compatible to Eric's excellent DSLstats which offers more graphing options.