let me put the sum of what i'm running, or not, at any given time, and what causes the problem all in one place.
my site runs about 25 plugins at this time. prior to this wingding unicode issue, the only problem i'd ever had is the All In One SEO program somehow causing occasional 404 errors on pages that were valid. It took some digging through my error log to start narrowing that down, but once the trail was sniffed, it did lead me to that plugin, and disabling (and deleting) it solved that.
wordfence had been installed as a security plugin, along with ithemes security for about 6 weeks. while digging on the AIO SEO program, i decided to dump ithemes security, too, and just go with wordfence. so...wordfence was the only security plugin running on the site for about 2 weeks. it does security in conjunction with our CDN company (incapsula) and i've been pleased with the results.
while looking through wordfence one day, i ran across the Performance Setup again, which i'd seen when installing but had left alone while getting all the bugs out of the site. i decided to turn on the Falcon Engine at that time. no other caching plugins were present at that time. the site went down, unbeknownst to me as i did that just before leaving work.
the next morning, the solving and return of the problem led me to the Falcon Engine and we left it off. as wordfence states that the Falcon Engine will not work in conjunction with W3 Cache, i decided to try that plugin with the performance part of wordfence turned off. it brought the same result.
i disabled W3 and we left that alone for the remainder of the day yesterday. this morning, from home, i set wordfence to basic caching to see if that worked. i saw nothing improper in the half hour before i left for work. after an hour drive, and approximately a half hour into my work day (2 hours from the change) the problem was back, fixed by removing the basic cache setting.
the StackExchange link posted by Tim above seems to have plenty of evidence (or as much as i can find anywhere about this problem) pointing to cache plugins and specifically to gzip issues. i am not anywhere near versed enough in the complexities of how gzip works, how the headers are created or passed to browsers, or anything like that. it just seems to me that this might actually be the problem.