Personally, I'd check the air filter 1st, make sure it's clean. While in that area, check for binding in the cables or anything blocking the throttle linkage. Are you getting a CEL? maybe the cat is starting to plug up. How many miles on the ol girl? Could be exhaust is restricted in some way. In theory, the fuel filter could be going...Need more info I guess.
Hows it idling? Last tune up....does it have Oil?....Tranny fluid ok?....Clutch slipping (if she's a stick)

 

Yeah, we replaced the air filter and sometime this week my boyfriend's doing an oil change and replacing the spark plugs (this was planned before it started not going right). He's also replacing the oil filter and we're picking up a new fuel filter today or tomorrow (as well as that sensor). I know it's had a tuneup recently (within the past 2 years)...I just don't remember when. The car belonged to his parents before we bought it and his stepdad did nearly everything himself. I do remember him having to take it to a mechanic shortly before we got it, but I don't remember why. It has transmission fluid and oil. He checks all that frequently. It idles like **** - when we stop for more than a few seconds, my boyfriend has to put it in park (it makes a lot of noise and shakes a bit) and it's just gotten worse. It's done that since our trip to Las Vegas in July. It's actually ran crappier since that trip (everything was ok until we got some bad gas and then it started running like crap. We couldn't use the air conditioning at all). As for miles...it has a lot. We all live roughly 40 miles from any town and for the previous 2 years before we bought it, his sister had it and every weekend was driving it from her house to his parents, which is about 200 miles one way...so it has been and continues to be driven a lot. The check engine light flashes on and off and has for years...no one has been able to figure out why. The seatbelt light is also always on, even when the driver is buckled in. There's a piece of tape over it because his mom got sick of it flashing at her all the time.

I'll show him this post and have him look at the areas you suggested as well. Neither one of us are really what you'd call car people and for what he knows, I know even less (although I am learning). Thank you for your help and suggestions.

 

And you can go to Autozone and get the codes read for free. Report them back here.

{Air filter and cat would be my first guesses too. But try loosening the gas cap; replace with a new OEM one if that fixes it.}

 

It could be a blocked exhaust. A clogged catalytic converter or muffler with collapsed baffle inside. Happened to me one time with a Civic.

 

So it idled like crap after a bad tank of fuel and is now worse? I'd change out the fuel filter and add a can of "Seafoam" May have a clogged fuel filter, or at the very least, gummed up filter. Could be a dirty injector also. The Seafoam may (and I stress MAY) clear up any injector issue. I had to pull the injectors of my 09 Sx4, which was easy...on that engine. Haven't really checked how involved doing so on the Rav.

 

Any news???

 

Check the throttle position sensor. An easy test is to unplug it while the engine is running, if it starts running better it's the TPS, Throttle Position Sensor.