I used to get this in the area a fly, alot worse actually, the cloud deck would be like 800 ft. It turned out that I had a high amount of bufferbloat, once a took care of that the clouds started to appear in the sky again.
So it could be your getting alot of resends of your data because your modem isnt able to process it fast enough, and the sim has to wait on the packets to be resent.