truck keeps loosing power

[sierra83]

ok so i recently changed my alternator and got a new battery but it seems like my truck keeps loosing power at night is when it gets really bad when i have to run headlights i dont know what the problem is i was thinking that it might be my wires because the voltage gauge goes up and down while im driving

 

[crazy4offroad]

This is probably related to this...
http://www.gmsquarebody.com/forum/showthread.php?t=3414
The wire for the dome light at the back of the cab runs under the carpet and may have grounded out.

 

[wheelerboy12]

i had that problem with my truck right after i bought it, replace your battery cables that should do the trick, if not then check the wires from your alternator.

 

[Jims86]

i had that problem with my truck right after i bought it, replace your battery cables that should do the trick, if not then check the wires from your alternator.

Check and clean all chassis grounds too. Make sure the battery terminals are spotless.

 

[HotRodPC]

I'd check out that dome light wire for a short for sure first. Eliminate that as a possible cause initially.
Then, where did you get this alternator? I've heard of people buying alternators at Auto Zone and O'reily's and it taking to the 2nd or 3rd one to get a good one.
And, by all means check chassis grounds as Jim said, and you might even consider just doing the BIG 3 upgrade while you're at it and you'll KNOW you have good grounds, and solid power.