On the festool dry wall sander the machine starts then stops the red light stays on I have replaced brushes any ideas

Unfortunately, many of these made today have a short life, and require discarding when they fail.
You might try to get brushes on "e-bay" or on Amazon with your model number.
Also, depending on the manufacturer, some sanders do not even have replaceable parts.
God bless your efforts.

depends how important it is to vacuum the dust, most generic brand sandpaper discs will have different hole placements for the sander and the vacuum cant suck the dust up. if you are getting swirl marks from the edge of the sander, is the disc too small? a generic brand that is slightly larger may work but try and keep the sander flat or the excess disk can cause a little trouble with the finish.

If you have shorting again by trying replace the capacitor but if it is not working try to turn a little the fan insite there is a collector burned out the commutator of the armature . if the machine was too hot replace both the armature and electric field if the lasting is not to great , ask yourself is my machine not to old for repair ?? because of the costs of the spare parts and the housing and remaining part. try out if the machine was not running this connect the cable again switch it on and carefully use a small screwdriver through the ventilation slot over but do not be surprised if he comes quite sparks and run.

The electric statorfield can on one coil side be broken

Autostop in carbon brushes > replace carbon brush if there is no wear problem on the armature commutator.
the Other defect can be the electronic print. or the powerswitch is clogged with dust between the contacts

its possible that the mains lead is breaking down, when it stops give the cable a wiggle and see if it starts up again, if it does then just replace mains lead

http://ekat.festool.de/login/jsp/main.jsp?doAction=start&autologin=true&userId=Kunde-FT_NL&password=Customer&design=festool

If it is not the plug in the wall socket being loose, then the most common is the motor contact brushes.

the winding is bad somewhere check withmeter

Control the coil contacts on both electric field contacts power cable and quicklock connector