For some time now, my '78 Spider has been popping out of 1st gear if accelerated hard or when coasting in 1st. If I am taking off from a stop light for example, and must accelerate hard to merge, I have to keep hand pressure on the shifter until it's time to shift to 2nd. There are no other tranny/shifting issues. Any idea what's causing this? Easy fix?

Tranny going south?
TIA.