when car gets hot after maybe 30 minutes, the gearbox slips into neutral while moving nessitating pulling off road. Car will go again after 10 minute stop
When was the last time the transmission fluid was serviced?
What kind of transmission do you have? Manual or automatic? We can’t know if you don’t tell us.
At twenty years of age and if this is likely an automatic it could be that it’s just near the end of it’s service life and dying. A fluid change might help buy some time. Or not.
If the fluid has never been changed then toast comes to mind.