I've been having a problem with NPCs I add to the game randomly dissappearing. They will sometimes show up when I first load the game, and then sometimes not. Then after I fast travel and return, they completely dissappear.

I have no idea what causes this...I think it has to do with the AI package. I'm too new to post a link, but if you search Youtube for "Dissapearing NPC's in Skyrim" you'll see a video of the problem.
Any help would be appreciated.

how did you make the npc's? if you copied another npc to start from then it's likely that they are behaving as the npc's they were copied from

I created a new npc, and followed the others as an example.

Bad Navmesh? Is this new npc also in a new area?

Sounds like the navmesh bug. If you're expecting them to navigate new areas, that's going to fail in spectacular fashion. Also the same result if you edited an existing navmesh, but at least that just reverts back to its vanilla state which isn't so bad. Unless the spot had no navmesh to start with.

I had a similar issue when I added a hill over top of existing navmesh and placed my npc on top of the hill. Turns out he was inside the hill on the existing navmesh. Try TCL-ing and flying around to see where the NPCs might be.

These are new NPC's in a new area...I've already added a navmesh to the area, and edited it. My guards will work some of the time, and walk around their patrol routes...other times they just stand there. Sometimes there are 3 of them, sometimes 2, sometimes one. Everytime I fast travel there...I get a different result.

I'll have to check into this. My town is built on top of the water in the middle of the lake just West of Riverwood. While I did an area select and thought I deleted all of the old navmesh, when I generated the new one, it may have created areas underneath my city, and my missing guards may be going for a swim under the floor boards.