Sudden Transmission problems - Help!!?

from your description it sounds as if you have lost the friction clutches for second gear. if you say it does it even when you shift it manually then the clutches and bands are worn out. starting out it should shift from 1 to 2 about 15-20 mpH then on up until you reach about 45 mph.. you have a turbo 350 trans in your 1500 chevy. and with that many miles it is just worn out. if you know how to rebuild it go ahead and do it. if not take it to the local shop and have them do it. your looking at approx. 2000 with labor.