Sounds similar to my trans. When going down a steep hill and using the trans instead of the brakes to keep the speed down, my transmission would jump out of 2nd gear - usually, not in a very pleasant way either! I put a 5-speed in and took the 4-speed to a reputable transmission guy in the area. He changed some of the worn synchros, but he thought the problem was the sliders, so he changed those to what he referred to as "hook" sliders. He said that these were designed to prevent the trans from slipping out of gear. The 5-speed worked ok around town, but fifth gear decided to abandon ship 1200 miles into a trip to the GS Nationals, so when I got home I put the 4-speed back in. Turns out I can now use 2nd gear to slow the car on steep hills and the trans doesn't pop out of gear, but now if I'm coasting on level ground, the trans will quietly slip out of third gear. I mentioned this to the transmission rebuilder, and he'd never had that problem after putting in hook sliders. He said if I pulled the trans out again, he'd take another look at it. I told him that it was just enough of a hassle to pull the transmission that I'd drive as is for now, and just make sure I held the shifter in 3rd when coasting. While waiting for better weather, I thought I'd check the adjustment on the shifter just to be sure it is getting full throw going into third - hoping to do that in the next couple of weeks. If I find anything noteworthy, I'll add it to the post.
I haven’t read this entire thread in a while so sorry if it is already suggested, but Check your shift stop bolts in the shifter in case they worked loose or Need adjusting and your shifter is causing the problem of popping out of gear on deceleration ?
