That sounds like band failure to me. When it attemps to shift but instead neutrals out there is an issue with the clutches or drums. The od band in these transmissions are small and prone to breaking, especially if the car has been run hard and had a lot of 4-2 downshifts and 3-4 at wide open throttle. Drop the trans pan and check for clutch material to confirm. There will most likely be material all in the fluid and on the magnet.