All shifts okay except 3-4

[Tpawlik]

I think I finally have all the vacuum leaks in the engine compartment fixed. My1-2, 2-3 and 3-4 shifts below 2,000 rpm are very smooth, but a little soft.
1-2, 2-3 are quick and firm, but not too firm, just right, 3-4 flares a little at 2,000 to 2,500 rpm. WOT are all quick and firm with no flaring. The transmission hangs on to 2-3 too long and then thuds into 3rd when creeping along in traffic. I just changed the filter and all the fluid using Valvoline Max Life Dex/Merc fluid. No change in the shifting from the fluid/filter change. I have the VCV set per the manual. If it ever stops snowing I will hook up a vacuum guage for concise readings. I have 10 hg on the modulator line with a Mity Vac, but it leaks down quickly. Will slackening the Bowden cable help with hanging on to 2nd gear too long? And what can I do to eliminate the 3-4 flaring under medium acceleration? I have not touched the Bowden cable or the modulator on the trans.
Thanks.