transmission downshifts to neutral/3rd at high rpm

[cho]

hi guys

my transmission (722.303) started to act up a little bit yesterday
shifting is fine until working temperature (10 miles or so...)
but when worm it shifts from 4th to 3rd on high revs and no matter what I
do it wont up-shift to 4th... in a city driving I did not notice this as my route is usually under 5 miles...but now on highway it is disaster...45 mph and lock

tried today again...same thing....

until worm all gears are fine 2-3-4....bang no 4th...neutral...let go pedal..3rd. no more 4th.

checked bowden and oil...all fine....

what you say...vacuum?

edit: yesterday it happened at 80mph,..today at 70mph.....


thanks