My armored trolls keep getting lost in dungeons and leaving my service. I know it's only 500g, but I'm not made of money.

That's better than Serena charging into an area like Leroy Jenkins while I am trying to sneak around and get stealth kills...

Money is REDICULOUSLY easy to aquire in Skyrim. And the Armored troll is uselss IMO. If you insist on using it....

Anyway, I do wish companions of all flavors had improved AI...

Is this a glitch, or is there a way to avoid this from happening?

It's just bad companion AI. Slow down when moving through tight areas, dont rush through. Should help.

Like, he disappears and reappears constantly and sometimes when they disappear they just stay that way. I know money is easy to make, but I like the armored trolls. They're decent as a companion, I think at least better than the war dogs you get near Markarth. They just disappear too often...

Nope. It's caused by the AI attack patterns that all Followers use. If they are close enough or are seen by an enemy, then they rush in like Dolomite to take care of business.

Unlike "D", It usually ends in their death or with them "taking a knee".

He is a troll... They aren't that intelligent... Lol. svcks though, its most defnitely just bad AI pathing.

It's like they're overcome with 'roid-rage or something. lol

Serana would do this too me in the Soul Cairn all the time. She would be right behind me then magically turn invisible only to show up behind me 100 yards down the road. Never really bothered me, but, it did seem weird to just watch her disappear and reappear at random.

More likely they try and force themselves through a doorway they can't fit into and get stuck inside the rocks so they can't move... HURR DURR.