Sometimes this can happen from external sources. A faulty MLPS (manual lever position sensor, in years past known as a neutral switch). The MLPS tells the processor what gear the selector is positioned in. When it acts up it can cause mysterious downshifts (usually from 4 to 3) and more rarely but sometimes a mysterious shift into neutral. I have had it happen to me with the NGS tester installed to monitor it. I was surprised to find what it was.
I have also seen this happen with a wiring problem that interfered with the VSS (vehicle speed sensor).
I don't know what is happening with your car, but these are worth looking into before doing an overhaul. If you have an external problem, an overhaul may not resolve it.
