Same load is getting higher and higher in velocity each time the brass is reloaded.

Could also be a carbon ring. Temp spikes aren’t usually that much..... maybe 20 FPS. Carbon can cause huge spikes in speed.

I agree with reloading all day, I've spoken to some bullet manufactures on this subject and deal with this situation myself. My tests I've done so far included settled powder in case vs shaking the powder prior to firing showed a significant change in muzzle velocity. Shaken powder showed a drop of about 60fps on average but a massive settling of my SD. Settled powder rose from my "standard MV" and velocities increased throwing out my SD. But so many factors change, how much copper in the bore will drastically increase your MV, different lots #s of powder, bullets, and primers change this, brass hardness, donuts in the case neck/shoulder junction, improper neck thickness, weather changes in temperature vs burn rates. With many other factors as well. Your best bet is to start measuring everything, chart it out and then look for discreptencies. I think because of all these variables so many people cannot give the right answer without all the facts, we as reloaders can only give factors which showed to most change which was temperature.in my experience a compressed load gave less variance because I didn't have to shake each round every time. A sniper taught me that one. Sniper 101 has very good insight on the things that change your velocities and learning the subject could help you isolate variables. Good luck man.

Temperature spikes due to weather. It’s not uncommon. Even a couple of degrees difference can make an impact.