Losing power on my 99' toyota runner up hills? Anybody?

[Chase]

Ok. I have a 1999 toyota 4runner that I bought about 3 years ago. My check engine light came on the other day ( It pulled a system too lean in bank 1 ) I did a little digging and it says that it is usually a mass air flow sensor. I had this same problem about 6 months ago. I pulled the MAF sensor off and cleaned it ( I have a K&N air filter so it tends to get dirty ) I will clean that but I think there is something else that is an underlining problem. When I am going up hills ( even small ones ) I am losing a ton of power ( on a normal hill I cant go over 45 MPH ) My RPM start out normal, but as I started to go up the hill I lose all the power and my RPM shoot up to 4-5k and I hear almost a knocking noise. I have been told that it might be my CAT or a vacuum leak. I dont think the MAF has anything to do with this problem because the problem hasnt gotten worse since the CE light came on. If anyone could help me, I would appreciate it so much!! Thanks