A test for a free wheel slip or a free hub slip. When you get into a section of pavement or dirt and you get into a big gear pedal backwards. Give a couple of revolutions forward then go backward's. If the free hub gearings are going out you will know, same goes for a freewheel. You will notive that the chain will not want to go around the cogs, and start to fall into the chain stay. No big thing though easy to fix.
Another thing, your rear hub bearings could be too tight. You can adjust them from the non drive side (left) easier than the drive side. Loosen the fixing nut, and back the coe out about a half turn. Then while holding onto the cone tighten the fixing nut back. Check this as well.