I have found with the 7 series that boot-loops are commonly caused by:
· The top flex cable.
· The home button flex cable connecting to the logic board.
· Audio IC. I have found if the audio IC is the problem it will eventually boot up.
· Baseband CPU from a botched audio IC repair. Remove the sticker and see if it has had a repair attempt on the IC.
If it has a prior repair on the audio IC it can also affect the Tristar IC as some of them are under filled. Also squids have a habit of causing boot-loops so I would suggest using a known good battery to rule out the squid.