The freewheel pawls are hardened steel and can be a bit brittle. Skipping under load is a symptom that the pawls may have developed a bit of chipping on the face that engages with the freewheel body. Hammering into power mode from freewheeling may cause this; however the skipping then re-engaging is likely to do more damage.

Skipping under load is worrisome and potentially dangerous. It always gets worse.