i am certainly not dogging your tranny so please dont get the wrong idea. i had a 5 speed in mine that did the same thing in 1 and 2. my brother and i tore it apart to see what it was. turned out to be a little end play in the main shaft. main shaft holds 1,2,3 and 4th gears. it allowed it to wiggle enough to jump out of gear. cheap fix but complicated. all it needs is a shim in the main shaft. but it is a bastard to get the trans apart. and a new gasket set.
The path of the righteous man is beset on all sides by the inequities of the selfish and the tyranny of evil men. Blessed is he, who in the name of charity and good will shepherds the week through the valley of darkness. For he is truly his brothers' keeper and the finder of lost children, and I will strike down upon thee with great vengeance and furious anger those who atempt to poison and destroy my brothers and you will know that my name is the Lord when I lay my vengeance upon thee.