1994 4L80E or 4L60E acts up with speedo

[Cobalt SS Dude]

Not sure exactly which model is installed but 7.4L gas and 4x4. i think it;s the 4L80E.
truck has just started to act up and trans will jump into 2nd and at the same time the speedo drops to "0"
any pointers on were to start to look?
thks in advance