Hey, long time listener, first time caller.
I've installed a 4.2 kw solar array thusly:
20 kyocera 210's in 5 strings with 2 midnite combiner boxes (with separate lines up to the inverter stack). Which means 3 strings on combiner box, two on the other.
2 xantrex 150-60 cc's
1 6048
8 rolls s-530's (400ah at 20hr rate, which is pretty good, I never push back more than 3800 watts)
4/0 AC lines back to the meter (the solar stack is in my shop, about 250 feet from the house)

To get some basic stuff out of the way, I bounce the batteries down about 10% once a week (checking amount out vs. voltage via a pentametric), eq+h20 checks once a month.
Water usage is nominal, my float target is 52, sell is 51.8.
The overall system is a DIY (my second), but it did pass a rather tough county inspection (still working on the utility co rebates, which is a different story)

It's been in for about 4 months, was running great during the summer, averaging about 22kwh harvested a day (18 or so usable/grid sell).
BUT, lately, as it's getting colder, the 6048 has started doing two things:

Instead of a nice steady sell, it starts to bounce from sell to just grid connected (e.g. not selling) then back again. I've dropped the sell voltage to as low as 51.5 with no change. The timespan of the bounce is short, say 30 seconds selling, 5 seconds in standby, rinse repeat. The voltage never drops below the target sell set point (it usually runs about .3 above the actual set value). Further, this does not always happen. At first I thought it was only when the output dropped to around 200 watts, but today (which is extra cold, bts says 60F) it is happening at 500 output watts. Last bit of info, I have not seen it bounce when the sun is actually out and I'm pushing > 1k watts.

The second bit of strangeness happens at night. Used to be the 6048 would drop out of sell mode when the voltage dropped due to "night". Lately, sometimes (not always), the 6048 will stay in sell mode after sundown, which burns about 100 watts continually and is just not cool.

So, first thing I'm going to do is update the firmware in the inverter, it's 1.00 (bought it back in 08), dongle is on it's way. The list of bug fixes/improvements in the inverter fm release notes is pretty long and I read a post from solar guppy about improvements in sell mode searching, which sounds rightish?
The fm on the cc's is 1.05, though i don't know if the revision numbers track across xantrex products or not.

Anyway, any ideas would be very much appreciated.

Updated the 6048 firmware to latest and greatest and things are running smoothly again (actually coldest it's been last night, no problems yet), pulled just over 12kwh's yesterday, which is not bad considering the fog cover in the morning.
Couple of things I've noticed so far:
Instead of just dropping out of sell for lower power production the 6048 fluctuates about 150 sell watts with the latest firmware, slowly up, slowly down. BUT, since this is during cloud cover and the raw power is also not steady I'll let it slide (raw was pretty close to steady though...)
During bright sun the sell watts were a rock, moving very little. Today is full sun, I'll update one more time with anything interesting.