It doesnt sound like you have an engine related problem if it was only at certain speeds. It would be rpm related then. You don't have a clutch, so rule that out. If it was suspension, wheel, etc related, the problem wouldn't just go away, although perhaps something got caught underneath and when you slowed down it got dislodged. I would look to the something driveline related: tq converter, transmission, awd system,etc. Good luck.