Is there any history of twisting injury.
Pain on folding legs and climbing stairs suggest meniscus injury.
And it is associated with clicking and locking of knee.
Next Steps
Need to examine and rule out the meniscus tear.
Only after proper diagnosis we will know the root cause of problem and treat it accordingly.
Health Tips
start knee strengthening exercises.
static quadriceps exercise,
avoid sitting cross legged, use of commode chair.