Cadence randomly drops to 80-89 during steady runs

Since approx. three weeks ago I noticed sudden drops in cadence on my base runs and long tempo runs. I always keep steady cadence, run without stoplights and walking, have been working on 175-180 spm for quite some time. Nowadays my runs look like this:

Every run now is this same problem. I don't have a treadmill, so don't know if the problem persists there.

Needless to say, all running metrics are off because of this. Would be very grateful for any advice.