Chuck,
I realize you state you've pulled the top cover and "it looks ok" in there. But,... (and your not going to like this), it's not likely the 1-2 detent and spring are the problem.
It's your 1-2 synchronizer, you need to replace it.. A detent and shift rail can be very worn and the toploader will still not pop out of gear.
A toploader that pops out of gear under decel is a very common sympton for a worn synchro ring. I just replaced the 1-2 syncro in my Shelby for the same problem.
Many people confuse the blocker rings for the syncro, the syncro is the actual ring that sides back and forth and selects the gear.
The internal splines on the syncro ring are designed to be a close tolerance fit with the internal piece that is splined to output shaft. After you remove it, if you look closely, you'll see the wear pattern on the inside of the syncro. It'll look like it's polished and barely worn.
There's your problem.
I would buy the parts from Mark at Toploader Heaven.
regards,
Charlie