Alternator and battery replaced now my lights are flickering?

It sounds like the alternator is defective or you have a loose connection at the battery.

Here is a guide to help you check on the battery problem:

https://www.2carpros.com/articles/everything-goes-dead-when-engine-is-cranked

If the cables are clean and tight, I would swap out the alternator for a new one. Here is how, also check the connections at the alternator as well. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.