Idle issue vehicle idles at between 1,500 and 3,000 RPMs?

I have put a new IAC in, and it made a small difference but now we are back to the same issue. I found a vacuum line that was cracked and fixed that and no change in the issue. I’m thinking it could be the header and downpipe not getting a good seal at the flange. Vehicle when downshifting RPMs skyrocket. Even in first gear the vehicle will run at 2,000 RPMs and be pulling the vehicle along as if I was giving it gas.