The stress generally converges slower than the total energy, because the stress is normally evaluated assuming constant number of plane waves, which is a bad approximation.
The code has two options to improve that. The first trick is due to Michel Cote and goes as follows. After the selfconsistent calculation has been performed at normal cutoff, a second calculation with higher cutoff is done, but WITHOUT allowing the charge density to relax. This turns out to be still a very good approximation, but the second calculation is somewhat expensive. The cutoff energy (in Rydberg) for the second run is set with the keyword
polished_energy_cutoff.
Example:
polished_energy_cutoff 50
Default: switched off
The second trick is to modify the kinetic energy according to
Bernasconi et al. You can set the height A, position 
, and
width 
(all in Rydberg) of the step function that gets added:
with the keyword
modify_kinetic_energy A .
Example:
modify_kinetic_energy 20.0 10.0 1.0.
If you just do
modify_kinetic_energy on
a reasonable setting is assumed: 
, 
,
.
Default: modify_kinetic_energy off