4 speed slipping out of second gear

Of the clutching teeth,under deceleration,there are about 10 teeth that carry the load, two groups of 5 each, situated about 180* apart. After the trans has popped out a few times, those teeth can no longer hang on to the slider, because the faces are now sloped like ramps. I grind those faces back several thousands of an inch and slope them the other way. Then I do the same on ALL of the slider teeth. Now, ten different clutch teeth will come on line, and the SLIDER will hang on to them like stink on a skunk.

But the guys are right, this starts out with a linkage problem, usually the nuts having come loose. So you need to prevent that from happening again.
What usually happens is that after the nuts come loose, with continued useage, the external levers get sloppy on the studs. So now when you re-install the nuts there will forever after be a tendency for the levers to work the nuts loose over and over, and the shifter will not hang on to it's neutral gate.
The best cure I have found is to drop some blue loc-tite into the rectangular slots around the studs, before installing the nuts. It will harden in there like cement and prevent the levers from shifting on the studs.
But, I have had it happen that the locking agent wicks up between the stud and the cover, locking it into place, and now it won't shift at all. It has done this to me, even after placing the cover on the bench fork-side up. So the trick is to put something in there to prevent the wicking. I remove the internal levers and smear them with rubber O-ring lubricant, also smearing the tunnels on the external side. And wiping off the excess after re-installing them. This has worked every time.

Bottom line is, if it has popped out more than a few times, I doubt anything will stop it from happening again, until you take the trans apart and backcut the clutching teeth and sliders.
This only takes a few minits, and costs just gaskets. You might as well check all the other gears while you got it all apart.