Hello ,
This issue seems to be matching a PR :
PR 788550 [Confidential] - VPN Drops Intermittently Between SRX and SSG
But this was reported in 11.4 version and fixed on versions above 12.1 . But we may need to get more information from logs collected during the time of issue . And also considering the fact that 12.1R versions are end of life and support , I am not sure how deep we can troubleshoot from JTAC point of view . So my recommendation will be to upgrade to 12.1X44 and above version ( 12.1X46-D45 will be a good option ) so that these issue have be fixed and have reported .
Else if this is not possible , we can try changing the Lifetime to lower value from both end and check are we seeing the same issue . Reboot can temporarily fix this , but if we are hitting a bug , it can reoccure in future .