Över voltage spikes

Hi!
Been reading a lot on this forum since i bought my XJ 1997 in July this year. Its my first 4x4 vehicle so it's a lot of things to learn.

Anyway I'm having some troubles with voltage spikes in my electrical system. Sometimes it goes up to 16-18v sometimes it's for a few seconds and sometimes a bit longer, a minute or two. Seems to be totally random. But it never happens when standing still idling.
Verified with DMM, can also hear the heater fan spinning faster and lights shining brighter when it happens.

So far I have changed ground cables between engine and battery, battery to fender well and engine to firewall. Also cleaned the grounds (g105, g106) on the left fender. Also cleaned the contacts on the alternator.

What do guys think? Is the alternator or have my regulator gone south on me? Is there anything else I should do before I go buy either a new alternator or an external voltage regulator?
Is there some way to rule out one or the other?

Edit
Sorry about the title. Automatic spell check.

I have found that if the voltage gauge shows 19 volts there is connectivity problem, If the gauge shows 9 volts there is an alternator problem.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Copper wires should be copper color, not black or green. Do the same for the grounding wires from the starter to engine block, the PCM, the ground wires at the coil, and the ground wires at the oil dipstick tube. You must remove, wire brush, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

I have changed cables between battery and fender well as well as battery to motor block. Also between motor block and firewall. And between PDC and battery and battery to starter motor.

Cleaned ground points on left fender well.

Replaced my alternator, didn't help.
Replaced my PCM with a used one, didn't help.

I still need to the grounds on the oil dipstick tube yet.
But if that doesn't work I have no idea what to do next.
Change battery, fusable link!

check the continuity of the wire from the alternator to the pcm its pin 20 on the pcm might be a dark green wire.

I think I've solved it. Not sure yet but was out driving for an hour today and it worked fine. Started at 14.4v and then when down to around 14 after a while.
Replaced the fusible link with new cables and a ANL fuse and also replaced the ground wire from the alternator to the motor block. Also solder new wires from the PCM to alternator.
Thanks for the help!