Sorry to hear about the problems, especially when the machine is recently acquired. The new brushes may be an indicator but did they change the brush holders as well at some point? Used to be common in repairing the commutator and brushes/holders in industrial power tools. Guy's would buy and replace the brushes as needed but near the end of service life on the tool, when it needed the commutator turned and undercut, even new brushes would sometimes produce arcing and smoke. Good luck, I hope you get it sorted soon!