Next Steps to fix audio

I am running into an issue with a well loved VHS that I have copied to my computer. I have done some basic audio editing before, but the sound (on the hifi track) changes volume by around 5 db (down, and then back to normal) every couple seconds, for about 3 minutes. Is there an easy way to "bring up the floor" so that the quietest sounds are as loud as the loudest, for a given clip? I have tried normalizing but it doesn't seem to make much difference in this case, or I am not using it right.

It appears the data was there, but that the head had a hard time reading it in the spot on the tape.

I have attached a screenshot of the audio.

Any idea, at least what tool I should be looking into?

Thanks!

I'm not an audio expert, just an editor. There are a few here, so hopefully they'll have better advice.

Normalization raises the whole level until the highest peaks reach the top, so that'll only raise everything equally; the low parts will stay low, relative to the high parts.

The compression in dynamics might be closer, because it lets you dial down the loudest parts without affecting the low parts, you can then add makeup gain once everything is closer together. This reduces the dynamic range of the recording. It's meant for relatively short sounds, however, so its efficacy will depend on the length of time over which the clip volume falls.

But even if you manually keyframe volume adjustments, or record mixer automation and raise the levels by hand, you'll find that the recording will sound uneven, because you'll be raising the noise floor along with the good sound, every time you push up the gain.

So, really you need to be able to de-couple the quieter parts from the noise floor and raise the good bits to match the rest of the recording, without also raising the noise. I think you might try Fairlight's "noise reduction" plugin to accomplish this, but automating it might be a little tricky, and you'd probably get better results with a third-party plugin. I think a lot of people use izotope RX8.