Dave,
You said that it happened on Saturday right? But you ran last night. Does that mean that it was just the one time? It could have been something as simple as when you ran the stock motor it put to much resistance into the speedo and it got hot and re-set itself. It could be the RX pack didnt take a charge or maybe one of the cells died in that and that cause a big enough voltage dump for the ESC to reset. There are alot of things that it could be to be honest with you. If I were in your situation I would put a new receiver pack in make sure your batts take a "normal" charge, turn a motor that you run with all the time, and go out and see what happens. Also, it never hurts to slap a heatsink on an ESC Just In Case right? hehehehe the biggest thing here is did you change anything when the speedo died? Yes, you changed the motor and you also went to a larger capacitor, so go back to your original settings and see what happens. get everything back to the way it was before it all went south and see what ya come up with. Hope this helps alittle bit.
IKe