You are pushing your cable modem somewhere near its limit, your upstream reads 54dbmv, in actuallity the modem can do a little more but the reading won't go above 54.
So again everytime you add a device (amp, larger splitter etc) you are asking your modem to work harder, so being near the edge it will work sometimes and sometimes it wont.
If you can get the cable company to come and measure your power level entering your home you can do the math from that point, it is normally suggested to take that entry point and split it 2 ways one for the cable modem and one for all else.
This way any amps or additional splitters installed won't affect the modem.
If this is not possible do your best to keep any amps out of line with the modem unless it has an active return.