Shifter linkage problem TH350

[darkpino]

Hi!
I have a really strange problem, i already tried the search button on the forum several times but couldn't find anything usefull... (other than the paintings i got now :P)

I got a 1971 Suburban with a 350 engine, TH350 tranny.

Now the shifting fork at the tranny side sort of "falls out'.
I found some paintings someone made :P so i added them.


Basicly the thing where it says "piece that you have" is falling out.
When i'm leaving, pedal to the metal style :P at the traffic light it does it.
Maybe it also does it when i'm just cruising but i noticed when i'm leaving pedal to the metal it's instantly gone :P

Does any of you have had this before?
The spring and washers and stuff is already in place so i really don't know where to search for the problem?
Thanks in advance!

Martijn