'Fraid it does sound like a clutch issue. One similar issue another member had that would explain the intermittent nature of the problem was a clutch disc that had a piece of friction material break off and wedge between the surfaces locking it up partially.
When something like this happens it's likely almost worn out anyways, so there prob. wasn't much life left before it started slipping anyways.
Give it a couple short tries starting it in gear with the clutch depressed. IF this is what happened you MIGHT be lucky & it's just a small piece loose that'll drop out & the thing will work again for a while....