I have been using a Makita random orbit sander model BO5010 for years without problems until I replaced the pad with a Porter Cable 13910, sold as compatible. Now the sander takes off, ie: increases speed with reduced control and overheats quickly. It sounds like a bearing issue. Is this coincidence or can the pad really be the problem?

Just under the sandpaper pad is a bearing that allows the pad to work as an orbital, that is it can turn freely, independently of the armature. If enough sawdust builds up around or inside this bearing the pad will spin like you describe and this is a fairly common problem with orbital sanders. You need Makita part number 211236-8 or a bearing #6002LL. It's fairly easy to replace; remove the screw in the center of the pad and remove it. Depending on which is tighter, the bearing will either be in the back of the pad or it will stay on the armature. Pry it off/out of the sander. Reassemble everything with the new bearing and you'll be back in business.

I have had this happen twice (I use the sander quitder e a bit.) The sander pad needs to be replaced. Dixieline (Probuilders) stocks them or you can or order it on line at http://www.ereplacementparts.com/makita-bo5010-sander-parts-c-97_103_112.html .

Buy from one of the many on-line parts sellers. Or do a Google search and you'll find hundreds.

Use some loctite thread locker on them.

use a thin wrench to hold the spindle and simply unscrew (counterclockwise) the rubber disc. i've seen replacements on amazon, etc... [search:
Porter-Cable 13700 Random Orbit Sander Pad"

Never start the Sander without first placing it against the surface to be sanded. In free air the Sander will not osolate.

Just solved my problem with Porter Cable 333 orbital sander which just groaned. I thought the worst. I carefully took it apart on the garage floor and immediately suspected it needed lubrication as the motor turned hard. I used my flexible long plastic tube lubrication and put some on the bottom bearing shaft the best I could. I worked it around and around. I began to feel it turning easier. I applied more oil and kept moving and it became completely free. I put a drop on the top bearing even though it was loose. I put it all back together and it works like new. Lesson: do not look for the worst when a tool quits. Suspect the culprit to be lack of lubrication!!!

Usually there are 3 screws holding the plate to the eccentric assembly. The pad is formed to a metal plate and is sold as an assembly. You can go to Makitausa.com with the model number and get a replacement pad.

Big lots have them for sale, on occasion. I've purchased 5" felt there several times.

http://www.makita.com/en-us/Assets/images/Products/BO5010K/parts_breakdowns/BO5010.pdf

let me know if that doesnt help and i'll talk you through it. i'm guessing you just forgot the order the pieces go back together.