By investigating and finding nothing loose, you meant to say that everything was torqued to manufacturer's specifications, right?

Unfortunately, backing up to 25 yards will probably give you the mathematical equivalent there of the groups you shot at 100 yards. From a diagnostic sense, one of two things is probably going on here: Either your torque specs are off and/or something else IS loose, or your sight picture is changing.

In nearly 35 years I've never found a lot of ammunition shoot that badly. I have seen some very well-worn Kalashnikovs with standard sights rival that group size, but I don't think that's what you're dealing with here, either?

Before I got too crazy trying to find a load that shoots better, I'd run back through your setup with a proper torque wrench. THEN - I'd make absolutely certain your sight-in platform is solid, whether it be a rest or sandbags. FINALLY - I'd make absolutely certain my point-of-aim was consistent from shot-to-shot.