I had the exact same problems and it was the new alternator. The 145 amp would drop the needle at wot but slowly creep up during normal driving. I had it checked out twice and it checked out good so they didn't give me a new one. I told them their machine didn't rev up high enough to cause problems but till the day I still have the $200 POS in the shed. The stock one is working great.

 

could also be belt slip at higher rpm's.