Poor antenna reception on channel 13

I have a Radio Shack antenna (model VU-190XR) and I live in Southern
California (Orange County).  I mounted the antenna to my chimney a
year ago.  At the time I mounted it, I had reception problems on
channels 2 and 4.  I tried everything to fix it and I was not able to,
so I paid an antenna professional to come out and adjust it.  When he
adjusted it every channel looked great.  Everything was fine for the
past year, until three days ago.  I am now having problems with is
channel 13, which is so out of tune I cannot watch it.  The picture
and sound is absolutely terrible.   The reception on channel 13 looks
like it is not tuned in properly.  It reminds me of a T.V. with rabbit
ears that need to be adjusted.  I checked the antenna for physical
damage and everything looks fine.  All the elements are extended out
properly and nothing appears to have changed.  I don't think the issue
with channel 13 is interference, because the characteristics of the
picture do not look like interference.  
 
When I paid the antenna professional to come out last year to adjust
channels 2 and 4, he told me he "fine tuned" the antenna.  I remember
him telling me he had some kind of reception meter he attached to the
antenna to make the adjustment.  Do you have any suggestions how I can
make the adjustment myself to get channel 13 to come in clear again?
Also, about the reception meter, I checked Radio Shack online to see
if they have something like that and I did not find anything.  Do you
know where I could buy some kind of reception meter?  I appreciate any
suggestions.