I have had such a problem with a recent product release based on 0.7.0 that I have jumped to 0.8.0-rc.8 to run diagnostics on 5 out of fleet of 300. I have made all the String class removal changes other than in the handling of Particle function calls.
I am waiting for a reply from Particle Support about exactly how to read the diagnostics download. If anyone on this thread understands the column headings and values it would be great to understand them and especially know what tell-tales to look out for.
Initial impression is that 0.8.0 suffers less than 0.7.0 with cloud disconnections and also with wifi network disconnections. I believe the diagnostics rule out heap fragmentation - about 49K used out of 83K and stable (some fluctuation but no downward slope). No Particle function calls were made during the tests.
Attached here is one download with the most recent report at the top - system has been up for 8754 seconds, 1 network dropout. The system memory used went up to 52K - signal strength and quality no change.
Link to another thread with output from 0.8.0 diagnostics explained. https://community.particle.io/t/remote-diagnostics-feedback/42715/4?u=armor