1988 T-Bird automatic transmission not shifting right

I have a 1988 t-bird whose transmission keeps shifting hard. Not clunking when it shifts, but shifting at a lot higher rpm’s. It was rebuilt about 6 years ago and was doing just fine until about a year ago. At first it was just slight, but now it happens almost every time I drive it. First, it was from first to second, now it’s from third to overdrive and even more recently going into first. What do you think? It’s progressively getting worse.