The key word in Dr Dialtone's reply is *conventional*
If you're holding on a TG, the radio is still scanning all of the site frequencies, so it wouldn't be able to tell which frequency you want to enable IFX for and it won't do it for the entire site. Plus, only the control channels need to be entered and the radio steers to whatever channel number the control channel says the conversation will happen on, so that problem frequency may not be programmed in the radio to begin with.
You should have some luck using the filter settings if you haven't tried that or try the attenuator and see if that cuts down on the distortion/interference.
If this distortion is only on one TG, then it could be a system issue.
Mark