I'd say play around with repeats except It's don't think you can do that with the 350. Sounds like you're doing everything I would do. The real problem is some cable boxes just do not like or respond well to favorites no matter what you do. I'm not sure why that is since there's little difference between that and just doing it by hand. Maybe alternate between .5 and full second delays? To be honest it may be a losing battle but again it's the box not the remote.

Just thought, something you could try experimenting with learning your number buttons in by hand, pressing them for varying lengths of time when you send them in.