No first, popping out of second when hot?

Your adjustment is too close. When the aluminum bellhousing & tranny case heats up & expands, it goes all wrong. If you did the adjustment yourself, try doing it when the engine is smoking-hot. Most likely, all that'll be needed is to change the length of the shifter rod near the left side halfshaft. Another thing that affects shifting is a worn-out trunnion support under the left header. With a handful of u-joints in the shaft to detour around the engine, the trunnion support needs to be pretty tight. Most aren't, being 30 yrs old with zero lube since the car left Italy. And sometimes, broken motor-mount bolts in the block can affect shifting. Check those 3 possibilities & let us know how it worked out.