I have a 1/4 sheet random orbit sander, now 3 years old, been working fine.  Went to use it today, and it just jumps all over the place, eventually tearing the paper off.  Since it was fine two days ago, this is puzzling.  Slip a gear or sumpin'?

That is odd. Maybe dirt build up? Compressed air to blow out the mechanism?

Some of them use a small counterweight to cause the weight and circle to wander out of the perfect orbit.  It is possible that it is stuck and needs to be free.  As Lew suggested, maybe some pressured air.  If you can see in the air slot clearly, you may be able to use an awl.  Please ensure that it is unplugged while doing this.  If there is no weight, then I would not poke around in it.  

1 hour ago, PeteM said:

I have a 1/4 sheet random orbit sander, now 3 years old, been working fine.  Went to use it today, and it just jumps all over the place, eventually tearing the paper off.  Since it was fine two days ago, this is puzzling. 

Did you change the paper?  Mine did this and when I went to change the paper I found a sliver of wood had gotten under the paper on one edge.

Changed paper 4 times (I'm somewhere between patient and nuts), it ripped it up all 4.  However, I'll look at the pad again:  maybe something is in there.  thx

Take the pad off of the bottom and look at the oscillating mechanism.  I have torn my Porter Cable ROS down a couple of times and replaced the bearing in the unit that give the oscillating motion.  When the bearing locks up it will jump.  A 6204 sealed bearing as I recall.

Turns out platten (the plastic square piece just south of the bottom bearing) is really warped.  No apparent reason for that, particularly given it worked fine just a couple days before.  Found a replacement part vendor (about $15).  Thanks for input.