my old sunbird did this
first gear was gone when it got warm, i sold the car as is as it had 220,000 km on it, orginal exhaust, battery, 2nd set of rubber (bald) but i paid 1200$ for it, lasted 3 years and all it took was oil changes, only had brakes done once at 140,000 km
someone suggested to tranny fluid filter could have been clogged, but i think first gear was just gone, it was an atx, rev up on acceleration, then id get a bang and start moving when it shifted to 2nd
