Vacuum goes on and off while vacuuming

Question

I bought this G4 used and it does fine when vacuuming straight back and forth but if I touch it against anything (ex: bump the couch) or pull it backwards at an angle while vacuuming it turns off. So to narrow the problem down, I knock the motor casing with my hand it turns off, knock it again, it turns on. Do you have any idea what this could be so I have a place to start?
Thanks in advance!