1998 jimmy power loss(please look Merlin)

Started truck this morning(garaged). Truck started ok but after about 10 minutes of warm up, I started out and noticed lights dimming,guages not working and power going away.....3 miles later after stopping and trying to take off truck stalled..dead battery. Would jump ok but once cables were removed,back to minimal power and stall again on take off. Had towed to shop where they charged and checked battery. Said battery is OK. Drove home 25 miles and charging and running great. When battery was dead I noticed a clicking sound like a relay or something continuously coming from behind glove box. Sound is gone now. Alternater maybe intermittedly working? When I start truck has low voltage reading until I rev it up high then goes and stays at good charge. Should I start by changing alternater? Or should I look someplace else for problem.....1st time it's happened to me.

When you get this one figured out let me know. Mine just started doing the exact same thing. I think it's a short but can't be sure.

Yes it was the alternator. I also bought a new battery as long as alt. was being changed. ALso when you replace check the battery cables for
corrosion and fit and check the terminals at starter for cleanliness. Mine
where filthy dirty. After changeout gauge goes to 12.5 to 13 volts and
stays there no matter what the engine speed or temp.

NowhereGMCMan