I had the "x" problem, too. Maybe I still have.
Here is what I did and tried:
1. Undervolt to ~0.85V with pencil mod, everything fine on various MHz-Settings, no x´s.
2. Undervolt to ~0.78V with pencil, on 2 of my ants the x´s show up. Usually the first 8 in first row, and sometimes even more, both rows. Sometimes I was able to get rid of them by reboot, sometimes they disappered alone. After a lot of trying clocks/timeouts from 162MHz up to 240MHz I was about to give it up. Nothing worked properly. Some days the x´s vanished, then show up again randomly. The pool hashrate also showed that the affected miners ran okay for several hours, then hashrate suddenly dropped from ~120 to 100 or even 80 GH/s and stabilized there. After all, I thought it was the pencil, because I was not able to get exact the same Voltages to all ASIC-sections of the antminer.
3. I removed the pencil, and soldered 4.7k resistors in parallel to our R3 and friends. Now I got a stable 0,776V (Load) - 0,779V (Idle) at each section of the hashboards. The Antminer is pulling now ~10,5Amps @ 11,97Volts DC and is hashing at ~120GH/s with 237MHz. So far so good. (Before it pulled ~13Amps with the 0,78V pencil mod) But when I moved my rig into the garage in november, the damn x´s got back.
Random drops from 120 to 100, on the other ant from 120 to 90 GH/s.
Temperatures in garage were around freezing, and I got it to barely 10 Celsius with the rig (4 Antminers total, and some Litecoin ASICs) running 24/7. Good to work some hours a day on my cars. COOLING INFO: even at 10 Celsius the fans ran so slow that you were able to count the blades on them.
ASIC showed around 20 Celsius. Nice silence.
4. Since december the rig is back in my house, in my living room as a little helper to cut heating costs. Suddenly the x´s disappeared after 2 days of (intermittent) running! (I need to shut the rig down when nobody is at home, to avoid risk of fire.)
Ok, so what could have caused the x´s? I believe it is only temperature and voltage that can cause it yet. I think the capacitors on the boards are aged, and some of them fail in cold conditions. Especially with voltmod, maybe the voltage gets very low under sudden load for a short time, causing the ASICs to crash... dont know.
And I do not know how well or bad the pre owners treated the miners, got all my 4 ant´s at the bay.
After a little research, I also think my voltage is a little low, maybe too low to power all the ants properly. Got 2 HP DPS-800GB with backplane that power my rig. The litecoin-section surprisingly NEVER crashed with this setup, at the same 12V rail.
Now I run 1 of the suspicious ants in the garage on a 12,34V PSU (instead of 11,97V from my HP proliant powermonster), to prove if voltage does matter. Actually running 4 hrs without x´s.
Maybe I got it....
Next step is to get closer to 0,75V... a 3,9k Ohm in parallel gets me 2,1k and a resulting 0,76V under load. I will report if it works fine and how much the miner is pulling then in DC and AC.
