Having rebuilt 10,000+ alternators...
I really suggest having this diagnosed electrically not by guesswork.
A shorted or open rectifier will cause an ugly ripple in DC output never mind chop alternator capacity by 30% or more. Bad rectifiers WILL destroy the stator if not repaired rather quickly.
Have a competent shop test the alternator at 100% output. A 120 amp alternator MUST be load tested for at least a half minute at full load, not a Kragen job.
Bad rectifiers or bad stator CAN cause the symptoms you describe. Professionals groan when a customer starts throwing around buzzwords. You may or may not be dealing with EXCESSIVE VOLTAGE RIPPLE due to a bad rectifier or stator. Save the alternator sine wave lingo for an alternator diagnostic laboratory. An otherwise useless term.
If the alternator is load tested PROFESSIONALLY and is found to be good...
Check to make sure the firewall to hood bonding STRAP is in good repair and tight. Wires are next to useless. A flat braided strap is used for a reason. But you say your instruments are affected. Keep reading...
Overwhelmingly when multiple instruments are affected on a single board, the instrument panel, the prime suspect is a bad ground wire and or connection running between the instrument panel circuit board and the interior firewall.
This needs professional diagnosis to cut to the chase...
Hope this helps
