Slow leg movement after walking longer.

Since last few months my mother aged 68, is experiencing something like slow leg movement after continuously walking normally for say around 0.5km or 1 km. Means at the start of walking she can walk normally like anybody but after walking some distance, she feels like leg has become slow and at some point her leg stops supporting to walk and at that point of time, she has to stop walking and take rest by sitting or standing somewhere ldle for 5-10 mins and then again she can start walking and after that again she can start walking normally for some distance and after that again she has to stop because her leg stops supporting or becomes slow so this sensation she has been feeling for some months now. It's not like she feels tired after walking longer or something like that and due to that she has to stop, not like that but her legs, specifically her left leg stops or becomes slow after walking some distance, same thing happens while climbing stairs or while standing for longer.