I rarely recommend a "mechanic in a can", but this is one time you might try a chemical additive that claims to dissolve varnish buildup on the valves. After many years of never hitting wide-open-throttle, I had a similar problem when I DID go wide open for a few seconds. Varnish had built up in the valve bores where the throttle valve hadn't been in a long time. I needed to manually shift to first gear when slowing down, and up-shifts occurred way too late. This was on my '88 Grand Caravan daily driver. There's 240,000 miles on that transmission and the filter and fluid have only been replaced once, a long time ago. As with everything else on that uncommonly reliable van, I ignored the problem and it eventually cleared up on its own. I drag a tandem axle enclosed trailer that's bigger than the van, and have not had a problem since that cleared up about 8 years ago, but I'm still hesitant to go wide-open-throttle again.