Yellow tang behavior ?

Im not sure about the tang but, your salinity seems a little low. Also you say you added a bag of live sand. Better keep a close eye on your params, this could start a cycle of some kind.

 

It's hard to say, Each fish of mine seems to have it's daily habits. The sixline acts acts aggressively to his reflection for 5-10 minutes at a time.
Three tangs that swim back and forth across the front of the glass all day in the same order. A single clown that does laps around the upper perimeter of the aquarium most of the day. A flame hawk that swims from perch to perch to perch in a certain order.
If your staying fish only then your salinity is fine, If your going to or have coral then you may want to raise it to 1.026.
Your pH is fine, it's better to strive for a stable PH than a higher pH.  Mine has been as low as 7.70 for days at a time with no negative side effects. Mines usually holds steady at 8.2-8.4 range.

 

It sounds stressed to me. My tang swims all around the tank, doesn't pace back and forth in the same area......whenever my fish do that, they are usually not well or something is bothering them.

 

My Yellow Tang for the most part swims around the tank, but like yours at late afternoon 7 night like now, he starts from the beginning of his cave hide out and darts real fast and stops fast like he is bucking his head up, and he will keep doing that for awhile.. He stays on half of the tank for the most part, normally where I have caves erected for security..