Targeted Run ?

easiest way is use your editor replace feature and change all the channel lines from update="i" to update=""
then for the channel or channels u want updated change them back to update="i" or update="f".
no global update mode needs to be set ==> <update></update>
save and run webgrab
only the channel u se to update i or f will update.
all others will be skipped and current epg is kept(unchanged).

when done use replace and change update="" to update="i".
u dont have to actually wait until then.
when webgrab runs it reads the config so u can actually do this while its doing the update and save it again.

edit ini and add delays.
add this below other site lines(retry setting may already be there on a different site line).
site {retry=<retry channel-delay="0" index-delay="0" show-delay="0" time-out="30">4</retry>}

killer is the show delay,it usually causes the error.
then index-delay
then channel-delay

all delays are in seconds.
so try increasing show-delay by 1 and see if error stops.
if u get higher than 3-4 then it becomes useless as it will take forever for it to comeplete.
time to find a different source.

maybe but how u going implement it?
where are you getting the 429,when u grab a bunch channels from the same site?
another option is to not grab them all at once(one after the other).
channel are grabbed in the order they are listed in your webgrab config.
mix the problem channels in with other channels.