ToyotaNoob

Hey everyone, I bought a 01 is300 a few months back and it's been a handful of problems ever since I got it, mostly suspension though. Anyways I replaced basically everything suspension-wise got it driving good, but now wether I'm in manual mode or auto if I put the gas to the floor it just makes a bunch of noise slowly goes up the rpms and really barely gets the wheels moving until it gets to about 5k rpm, then it shifts up a gear and is fine, but then if you're going any speed at all and put the gas to the floor it tries to downshift but just acts the same as when I take off. The only CEL that I have is for 02 sensor. So my question is: is it the throttle body? Is it burned up plugs? Or could it be as simple as my transmission is just slipping and won't grab 1st.
I have only messed with Hondas previously but my automatic Integra I had a while back was doing something similar and it was the tranny in that car. this stuff is all pretty new to me so any help would be appreciated, I've owned a ton of Hondas and was hoping to get away from all there problems by going with toyota/lexus but they seem to be equally as aggravating. Once again, any help is appreciated