2001 Jeep Wrangler transmission

My 2001 jeep wranglers transmission slips into passing gear at hiway speeds of 55 or faster only on hot days when it is warmed up. On cool days no problem. It is worse in cruise but does happen without. I let up on the gas and it shifts back in. It happens on a up hill grade or when I step on the gass too fast. I can nurse it up. The dealer had the tranny out and in [so they said] for a down shift into first gear that ended up being a throttle position sensor. Before that no problem. Any idea's. Fluid level ok