[mythtv] channel skew/offset in program guide

[Tom Metro]

I'm about a week into a new MythTV installation (version 0.20 via Ubuntu 
package mythtv-backend0.20-0.2ubuntu2 on Fiesty; using zap2it as data 
source) and I've observed that my program guide data (as viewed from 
mythtv-frontend or mythweb) is frequently showing a channel skew of one 
channel number for listings a day or two in the future. For example, my 
Dec 4 listings look OK, but on Dec 5 the first three channels list "NO 
DATA", followed by the 3rd channel's data showing up in the 4th 
channel's slot, and all subsequent rows offset by one channel row.

I searched the wiki, trac, and both mailing lists looking for a 
solution, but found none, however I did spot several other threads where 
people report the same problem:

MythTV channel listings skew problem. | MythTV | Users
http://www.gossamer-threads.com/lists/mythtv/users/205041

Can't get channel listings to stay aligned | MythTV | Users
http://www.gossamer-threads.com/lists/mythtv/users/186352

Channels mixed up | MythTV | Users
http://www.gossamer-threads.com/lists/mythtv/users/239470

Database question/problem | MythTV | Dev
http://www.gossamer-threads.com/lists/mythtv/dev/503


I got the impression from one of the threads that one possible cause for 
this might be mythfilldatabase having difficulty parsing the data from 
DataDirect. I altered my cron.daily script to log the output from 
mythfilldatabase:

# rcsdiff -r1.1 mythtv-backend
diff -r1.1 mythtv-backend
3c3
< su mythtv -c "mythfilldatabase --quiet"
---
> su mythtv -c "mythfilldatabase >> /var/log/mythtv/mythfilldatabase.log"


but so far nothing interesting in the log.

I'm looking for suggestions on what to look at next in order to track 
down the cause of this. I noticed when I run mythfilldatabase manually, 
it looks like it is calling wget, and sending the output to STDOUT, so 
I'm guessing there aren't any temporary files where I could examine the 
raw XML. Though presumably I should be able to access the DataDirect web 
service manually and retrieve the data to see if the offset originates 
there. Is there a debug mode for mythfilldatabase? Are there any 
particular database queries that might illuminate the source of the offset?

Also, I'd be curious to know what the mechanism is for matching up 
channel data from DataDirect with channel slots in the program guide. I 
would have expected it to involve the IDs (xmltvid), and thus a parsing 
error on one channel shouldn't offset the rest.

  -Tom