Need Toploader 1-2 detent and spring.

[Excaliber]

Yup, what Cal and Roadracer said, unlikely it's the spring/detent pin. One other possible is your shift boot. The rubber shift boot may be mounted a little bit to "forward" toward the engine. When the gear shift is pulled back into 2nd the rubber acts like a spring and puts pressure on the gear shift in the forward direction. It's just a little pressure, but enough to slip it out of gear under the condtions you describe. How about you remove the chrome ring to let the rubber boot totally relax and test drive again.

On the tear down be on the lookout for the tiny and often over looked "reverse lockout" pin. This is NOT like the other "detents". It goes in "sideways" compared to the others and slips out VERY easily during reassembly. If it falls out it will be possible when shifting into 1st to "drag" the reverse gear along and cause a "lockup" condition where the trans IS in 1st, but the reverse gear is partially engaged and the shifter mechanism "binds" BIG TIME. Then it's hard to get the shifter to move OUT of first, or reverse. Nasty little pin to install and keep in place during the rebuild. Careful not to use "heavy" grease on those needle bearings during assembly also. That grease won't easily melt away and the needle bearings will "starve" for oil when you first start to run the trans. Use some good "trans assembly type grease", with a low melting point.