Sounds like a transmission issue based on your second post. When my high clutch went out it had similar symptoms. I'd be driving along on the highway and it would feel like something was slipping since I could increase my rmps but I wouldn't go any faster. Eventually it couldn't even go into third or fourth and I ended up replacing the transmission.
Not saying this is what's wrong with your car but it's certainly a possibility and probably the worst case scenario.
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