Slipping transmission

[1967250s]

Last year when I bought "the monster" 1967 250s, I started a thread (vintage section) asking for info on different subject wich included slipping tranny and did not get a satisfactory answer.
Maybe some of you here may have some insight?

Here are the simptoms; it slips in all gears till it gets to 4th at 47m/h, once in 4th it dos not slip.
Once I tested it on a vacant lot, to my suprise all gears were working fine, it lasted for one pass only, then started to slip again, because the first is so short, I c'ant tell if it slips in first!
But I do know that under hevy load such as going up ramps in first it dos not slip.
Also, on hard acceleration it slips less, but still the 4th only ingage at 47m/h.

From what I have read, I'm suspecting 1 of 4 things :
1) malfonctionning silinoid switch (top of tranny)/idle/kikdown switch.
2) vacumm conduit blocked.
3) low oil pressure (worn pump, clogged valve)
4) gungk deposit in valve body (from siting to long?)

It is still winter here, so I won't be able to test you'r advice now, I will be doing those in spring.
As anyone of you hever tried a transmission cleaning agent? What was the result?

Thanks for you'r expertise

Dan