Diagnosis
This is classic short-cycling, and on a dehumidifier, the most common cause is a dirty or frozen evaporator coil. The coil gets too cold, a safety sensor trips, it shuts off, it thaws, and the cycle repeats.
How to Fix It
1) Unplug it and let it sit for 24 hours to completely thaw. 2) While it's off, clean that air filter thoroughly (see the other question on how to access it). A dirty filter is the main reason coils freeze. 3) Plug it back in in a room that's above 65°F. These units struggle in very cold spaces.
If That Didn't Work
If it's clean and in a warm enough room and still short-cycling, the refrigerant charge could be low (a leak) or the temperature sensor on the coil could be faulty. Both need a pro.
Heads Up
Don't try to chip ice off the coils. Let it melt naturally. Forcing it can easily puncture the delicate aluminum fins and cause a refrigerant leak.