Merry Christmas Eve!

About a week ago, my 81 300SD developed a delay when up shifting from 3rd to 4th gear. The upshifts from 1 to 2 and 2 to 3 are fine, and it downshifts fine, but that 3 to 4 upshift is delayed. ATF level is fine. Fluid is bright red, no burnt smell. It's not my daily driver so I haven't really delved into it. I'm thinking it could be the K2 spring, but I need to check the simple things before I dive in that far. I'm thinking to check the things that control the timing of the upshifts, so the bowden cable will get checked. I think the kickdown switch is ok because it would cause delayed upshifts in every gear if it was stuck. Am I missing anything else to check? I'm thinking perhaps vacuum, but if I remember correctly, vacuum controls the quality of the shift and not the timing. Looking forward to getting your thoughts.