Do you lose your speedometer or tach when this happens? It doesnt make sense that you arent throwing any type of code. If you are entering limp mode then there should be a code that signals the reason this occured. The fact that you did get a code saying speed sensor could possibly relate to the input or output sensors. This would cause your problem if they failed or were functioning incorrectly. However other things you describe lean away from this. The fact that there are no codes is very odd unless there is some type of mechanical issue. The fact that it goes away after it sits for a while could relate to fluid levels or temps. But the temp should cause the transmission to throw a code.