Does this only happen to RDP or are other Services that need to pass the SRX impacted?
Could it be that your SRX drops rdp because of a "timeout"?
Out of the Box this happens when there is no "input" for 30mins (default).
You can check the timeout of your session with this Article:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB21344&smlogin=true&actp=search
We often get calls by Customers, that their SRX is doing this - mostly because the timeout-value has not been increased.
You can set the timeout-value per Application:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28630&actp=search