r.barn

could be bad IAC ........ they cost $30 and take about 5mins to change

or low voltage from the alternator can make them surge a little like that
does it have an electric fan or underdrive pullies?

surging idle no big deal, can be fixed, dont let it hold you up.

Swirl

I cleaned out my IAC and Throttle body and this stopped my mustang from doing just that.

Swirl

disturbed471985

Tell him to take atleast $500 off. Then drive it home spend 20-30$ and spend the other 450$ on mods.

budman

Pop that sucker off and give it a cleaning and you should be good to go. Very common issue. I have been there with every 5.0 I have ever owned.

ruger311

yep for sure check and clean the IAC everyone has this same problem sometime!