I had a similar problem on a Nissan 720 pickup. I could shift into first gear, but the trans wouldn't stay in gear unless I held the shifter in place with my hand. If I took my hand off the stick, the trans would pop out of gear, and it only did it in 1st gear. After driving the truck for about 10 years, it got so bad that it would pop out of gear even with me holding the stick and putting pressure on it, so I finally broke down and had the trans rebuilt, and the problem turned out to be a bent shifter fork, which had been bent before I bought the truck. That bent shifter fork only affected first gear; second through fifth were all fine. So I think it's entirely possible that the downshifting problem may be a bad shifter fork too, and it also can affect only one gear, the same way the one in my truck did.
