I have a 2004 Lexus GX470. I am having this problem where as I accelerate, my RPM goes up to like 5/6 and my speed is not going up. If I back off the pedal and try again it sometimes will accelerate. It doesn't happen every time I accelerate, but has all of a sudden started happening.
It could be one of several things, all of them pertaining to the transmission. Whatever the issue turns out to be, it is slipping. Either something is worn out, or is not doing it's job. Just rattling things off, this is what I've come up with, but do not consider the list exhaustive.
First of all, it could be fouled transmission fluid. You can pull the transmission dipstick and check the fluid. If it looks brownish and/or has a burnt smell, this could be an issue. You could quite possibly get the fluid flushed, but this could provide other issues, depending on how often it has been changed in the past. If routine maintenance has been done, you shouldn't have any fears about changing it now. If it has never been changed, it may lead to problems. When you flush an automatic transmission, it can clean out sediments which are actually helping it to work. By removing these, you can cause the death of a transmission. Only cure after that is complete rebuild.
Second, it could be one of the servos is getting weak. The servos cause the bands to grip tightly. If one is losing it's power, it could be allowing it to slip. Getting a servo replaced is a relatively inexpensive item (when you consider a transmission rebuild). Along these lines, sometimes the computer can cause problems. You might want to have it checked to see if has a recall for transmission issues. A reflash can sometimes cure these types of problems.
Thirdly, it could be the pump failing. If the line pressures go down during operation, it could allow the bands or clutches to slip. If the transmission fluid filter is plugged, it could be exhibiting the same type of issue. Transmission could just be due some maintenance, but if the pump is going out, it would entail a rebuild.
Fourthly, your transmission could just be worn out and need a rebuild.
Something else which could potentially be the issues is, your car may be low on transmission fluid. While this sounds like an easy fix (just add fluid), it would mean there is a leak somewhere. Transmissions don't lose fluid unless there is a leak (it doesn't burn/eat it like an engine can engine oil). If you've seen any spots on the driveway or in the garage, this may be what you're looking at. In this case, it could be a leaking seal or a pan gasket. Depending on where the leak is originating from, it could be an inexpensive fix or moderately expensive (the amount of labor directly affects the cost).