Garageband unwanted voice echo

OS X 10.9.5, 2.4 GHz, 6GB MacBook Pro

Garageband '11 version 6.0.5, Blue Snowball Mic

I have been using Garageband for about 2 years to record live hypnosis sessions with my clients.

I first import a background music track (as AAC) and lock the track to keep it stable.

I use NO effects.

Audio resolution set at Good.

I then record my voice as I hypnotise the client, with the music track playing, and my client and I listen through Sennheiser headphones.

Here's the problem:

Sometimes (around 1 in every 6 recordings) my voice gets distorted part way through - sometimes 5 minutes in, sometimes 10 or 15 minutes in.

It sounds like I'm slurring and drunk, and at variable speeds.

I have to abandon the recording and start again.

I've done everything I can do not overload the harddrive: lock the track, use AAC import not mp3, no sound effects. But I don't think it's the harddrive because the cue doesn't change colour (remains white)

Could it be a loose USB on the mic?

Should I upgrade Garageband? (scared to in case it makes it worse)

Should I give up and try Audacity? (Aggggh!!) Get a new Mac? Become a plumber?

At the end of my tether - any help appreciated.