2000 Toyota Sienna XLE Minivan

I am the second owner since 2009. The engine was replaced with another used engine that had 70,000 miles on it in 2015. I have been and still am driving this car everyday, mostly highway miles. Never had any problems until now; it seems transmission is not doing its job 100%. Car stalls momentarily when you try to accelerate but will soon (2 sec) engage and move. It consumes more gas than before, too.