What powder measure are you using? A Lee 'mechanical' scale isn't the most precise thing. Ideally you'd have a quality 'beam' scale (older RCBS units were quite good) and a digital scale so you could throw a charge and then verify on two different scales to eliminate inconsistent powder charges as the issue. For example, many people use an RCBS Chargemaster Lite to throw their powder, then they re-verify on a beam scale before putting it into the case.
I'm sure folks are on the right track with the need to find a more temperature stable powder, but the variation you described seemed pretty large unless you developed the load at 5 degree Fahrenheit and then shot it again at 95 degrees. Figure one FPS change in velocity per degree of temp (in Fahrenheit) as a general rule of thumb for a 'temp sensitive' powder like R17 or R19. Getting more variability than that? Something else is probably going on.
Cases are all the same headstamp? Are you annealing? Do they have the same number of firings on them (i.e., all once fired brass or twice fired brass?) Are you cleaning your brass with steel media vs. corn cob media? A super clean case mouth (i.e., bare brass vs. a case with some carbon still in place) is more likely to 'stick' to the bullet, so as the ammo sits longer, you get more 'pull tension' than when you first loaded it.
Just spit-balling some ideas to go with the potential powder issues.
