Intermittently will not run when trigger pulled, juggling and turning shaft let it run. Is it brushes, commutator, or speed swithch. This is Hitachi DS14dvf, about 5 yrs old

Check the brush holders and make sure the brush can travel freely all the way down to the commutator.
Look down the brush tube with a light and turn the blade to rotate the armature - look for missing/burnt segments on the commutator.
Also, this has a soft-start module that can go bad - if the brushes and armature are good, switch rings out, and all connections are tight, I would lean toward the module.

faltering on a slide contact on electric field or switch, but in most cases dirth (dust) in carbonbrushes can be hanging somewhat in one or both brushholders.
Or a problem in the trigger switch itself dust and dirth have almost the highest priority in case of faltering by clamping carbon brushes.

Think you need to replace the motor brushes. As they wear they get shorter and the spring that pushes them up against the commutator is now at the end of its travel. Replace brushes or stretch the spring a tad.

you should only get voltage at the brushes when the trigger is pressed... is the tool constantly running or is it doing nothing?
If your getting voltage to the brushes and its doing nothing... check the brushes to see if they are worn out and not touching the armatures commutator. Replace them and the switch

brushes in the motor

The brushes are usually replaceable from outside the drill. Make sure each spring is properly tensioned. Make sure the electrical outlet is live by trying a working lamp or other item. Pull the trigger full on and wiggle the cord where it goes into the plug--if the tool turns on intermittently, suspect a bad cord connection at the plug. Just replace the plug after cutting the cord back an inch or so. You can test the speed control by a temporary jumper across the module which should allow the tool to run full speed. Make sure the reversing switch is ok. Hope this helps!

12 volt dc motors use brushes and commutators. One or both of your brushes may be making intermittent contact with the commutator bars, or your commutator bars may be bridged with carbon buildup from the brushes wearing down. To check brushes remove the slotted round caps and check them. Look inside the hole after you pull the brushes out. You will see parallel bars of copper. the spaces between all of them should be clear of carbon and debris. the coppers should be shiny.

the commutator might be dirty, try and turning the brushes 180 degrees and you might have to clean the commutator with a stone for that purpose to seat the brushes.

there is always going to be some sparking between the BRUSHES and the commutator. if it is excessive and loud or is really heating up the housing, there could be a crack in one of the brushes, the brush might be chipped or unevenly worn, or the brush holder itself may be burned cracked, or otherwise need to be replaced. they may LOOK okay but need to be pulled out to really know for sure sometimes. there is a small set screw in the housing that holds the brushholder in place.

while this could be the problem with the electric brake, it is more likely that there is a wiring connection loose, broken or burned inside the motor housing. it could be the switch but i dont believe the brake circuit runs through the switch on this model.

sounds like brushes not being pressed down to the commutator shaft. pull out brushes and clean the brass brush guides and then apply a light coat of grease to brushes. however, what exactly do you mean by "the fan is OK"?