The problem isn't with the 300, it's with the IR sensitivity of the two units your having a problem with. Move the flashers around and put the IR to the lowest setting on those devices. If that doesn't work, you could always put multiple flashers in parallel and the strength will be reduced even more.

And before you bash CIs anymore, realize you are now posting in a CI area of Remote Central and Rich_Guy hates URC and has done everything in his power to let everyone know that for the last month. I think I can safely say that most of us CIs are a little tired of hearing that, plus the fact a lot of DIYs keep reminding us that they don't need us. So if CIs haven't been as helpful in the last month, now you know why. If you don't believe me, please feel free to read all the posts for the last month.