My car revs while accelerating with little speed. What is wrong?

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So I have brought my car to the shop a few times now. First my car was stalling out when I would idle. My mechanic just put an additive in and said it was fine. Then it completely died on the highway. They replaced the crankshaft. Then my car decided it wasn't going to pick up speed at all and I found out that the catalytic converter was clogged (probably from the additive) so I got that fixed. Now when I drive it takes longer then normal to speed up. My car is an automatic but it's feels like I'm badly driving a manual. It will rev up between 3 and 5 depending on how fast I'm going then after a few minutes will drop down under 2. Any ideas?