It sounds like this could be a few things. Does it appear that the engine (tachometer) races when the vehicle is in neutral or only when in gear? How fast can you drive before the problem appears to go away? Is there a check engine light on?
How's your fuel economy? I once had drove my '00 Altima with a completely broken MAF and my symptoms were just as you had described. Car would start fine, but as the RPM's leveled off and I put it in gear the tach would race and the engine would rev without depressing the throttle. Also, if I accelerated to quickly my engine would "kick-back" and my tach would start racing again. So I had to find a happy medium and keep a close eye on my RPMs. I think that 40 mph was about as fast as I could go before my engine started going nuts. Another side effect, fuel economy was terrible and I noticed a build up of carbon on my rear bumper where my exhaust comes out.
The good news is that MAFs are easy to replace (assuming this is your problem) but it's not a cheap part. About $250 brand new. I would suggest first taking your MAF out and inspect it for dirt and debris. The MAF is located between your air filter box and the intake manifold. You can buy a can of electrical contact cleaner (made by CRC; $5 at Autozone) to spray off any debris on the sensor inside the MAF.
Good Luck!
