Blower motor won't stay on more then 5 minutes, and that's if I can get it to come on. When on, after 4 minutes or so, it start squilling and slows down until stops completely. Looks like the unit has been here awhile, motor interior very clogged with dust and dirt. Do I need to replace motor?

Probably,, watch it close,next time it starts and runs a few minutes,, get to it and feel the motor ,see if it get hot/very warm, the clogging and dirt probably from someone not keeping it clean and maybe no filter to keep dust out has cause this and it is/ has been overheated to many times,