F100 Duty transmission problem

Hello friends: I have a F100 Duty Model 2000 Simple Cab 4x2 Cummins 4 Turbo Diesel Mechanical Injection
in other countries call it F250 which is the same.
I have the problem that when he warms up I put him first and when I release the clutch he wants how to jump the change
is like something wants He does it when he is hot only, if any forist if he has had the same problem and can guide me I will be grateful thanks.

Hello Oscar, I fear you will have a problem in the gearbox.
I do not know the model but if the problem ..
when it is heated it is likely that the moveable of first does not hook well to have too much tolerance and by adding that the oil of the change is very fluid wants (jumping the march).
Do you only do it in first .... If you try to go out in Second, right?
If it is only in first and hot it discards everything peripheral and focuses on change.
All the best

Hello DDLCSA: Thank you very much for your answer, you know that I did the test of holding the lever with my hand and also does, what test could I do to get the doubt that the volume of first does not hook well and tend to jump the march, and it is only in first the sack in the second and it goes well.
I also change the oil to the box but the problem continues, put an 80W90 API GL3
because they say that the GL5 wears the synchronized bronze synchronized and here you can not get API GL4 there are GL3 or GL5.
Thank you very much greetings Oscar.