I just completed my printer today after a (too-)lengthy build. I did a few short prints (~30 minutes) and they came out fine. However, I started a print that was estimated to take 2.5 hours and noticed only a couple layers in that the extruder stepper was skipping. Weird. The temperature was dropping and had got to 172°C by the time I noticed. I tried to restart the print a couple times, and noticed that it was able to get to the starting temperature (195°C) just fine. I did a test and was able to hold the temperature at 195 with no issues for about 20 minutes. After the print began though it would drift down through the 180s and get to the mid-170s by the time I shut it off. Target temperature did not change the entire time. What might be the issue here?

I checked the electrical connections for the hotend and thermistor, and they seem fine; I knew they would be fine though, because it can reach 195° no problem.