When you deplete all of your data allowance you can expect speeds to be throttled to around 150 Kbps (about 3X dialup speed).
All Gen4 plans now offer the SmartBrowse feature that allows for near uninterrupted speeds if that type of activity is "low demand" ... no streaming, no large attachments.
It very well may be that you have an unknown background process running that is kicking you out of "SmartBrowse" and into FAP.
You can check to see if SmartBrowse is active by opening the Download Status Meter and see if the SmartBrowse "light" is active:
You can also purchase blocks of data known as "Tokens" by clicking on the "Purchase Tokens" button on the upper right of the Status Meter. That will offer the following options:
In the event you decide to purchase Token Data please post back here in the Community so we can show you how to "use" a token after purchase so the that data amount is transferred to your "Token Bucket".