Are you sending group messages as MMS (replies go to all) or bulk SMS (each recipient gets a separate SMS, replies go only to you)? If the former then the obvious difference would be that your group messages are MMS and the rest SMS. Of course MMS should not be delayed either, but they do require a data connection to send so if you have that disconnected it would delay them. Otherwise perhaps speak to your service provider (possibly they'll say they know nothing or that it must be your phone without doing any investigation, but you never know...).
I'm assuming that there is actually an hour delay, rather than just a timestamp being wrong? Because if it's always exactly 1 hour that would be a very odd thing for any technical glitch to produce unless it somehow involved a clock being wrong.