Upright Freezer Working Intermittently and Had a Frozen Drain Pipe

We have a GE upright freezer (model FUF17DPERWH) that is no longer cooling consistently. It started with noticing a puddle of water around it yesterday and discovering that there was water frozen in the drain. We defrosted it, and it then appeared to be working normally. It re-froze everything we put back in overnight. However, today, I noticed it had warmed up again. I don't see any ice in the drain this time. What could cause it to work intermittently like this?