That type of problem is most commonly caused by an old, damaged, faulty, or worn out battery pack.
If the E300 scooter is a version 1 through 19 then it has battery level indicator lights on the throttle which can be used to determine that battery pack's condition. If the number of lights on the throttle decreases as the scooter stalls that indicates that the battery pack is not able to maintain its Voltage under load and that it needs to be replaced.
If the E300 scooter is a version 20 or higher then it does not have battery level indicator lights on the throttle and the battery pack would need to be load tested to determine its condition. We have a battery pack load testing guide on this page: https://electricscooterparts.com/loadtest
If the battery pack winds up passing the load test then the problem is elsewhere and could likely be the motor or throttle.
Please let us know how it goes.
I have an e300s, the model that came with a seat but it had no seat when I got it. Anyway, with even the slightest incline (you wouldn't even know there was an incline other than it keeps stalling only in that direction) it stalls then after a minute will start but then stalls again. Extremely annoying. It happens more often after a fresh charge. Is there anything I can do without modding the frame?