4L60E Transmission problems

I have no clue whatsoever what kind of automatic transmission this is, but I have had a similar experience;

I once borrowed a mates automatic suzuki alto for the day and halfway where I had to be, it started falling out of gear i.e. loosing all drive. Rolling to a stop, selecting a lower gear (forcing it into that gear) or letting go of the throttle and bury it directly afterwards made it react again and off we went. In this case it turned out to be a corroded fusebox, something these altos are known for apparently.

I realise a Suzuki Alto is not quite a Chevy G20 van, but perhaps you could start at the fuse/fusebox, especially the ones under the hood.

Of course if it's a mechanic automatic tranmission then this all doesn't apply, but then you could start by checking the vacuum lines going to and from the inlet/transmission.

Probably obvious but you did check the fluid level with it hot and the engine running? I'de do a fluid and filter change before going much further, if nasty things are found in the sump at least you'll have an idea of what the problem was.

If it changes up when you give it a bootful isn't kickdown working the wrong way around, usually backing off the throttle when up to road speed makes them change up a gear?

I say fluid, or a faulty solinoid!

Get it fixed its as annoying as curse word!