Sounds like an Ultra. If it's weird, it's an ULTRA.
If it's NOT an Ultra, ignore the following.
If it IS an Ultra, hook up a serial cable and run the Gtools program, upload the channel data from the receiver and check along the left side to see the TP count. I'll bet you have like 1500 transponders saved. The limit is about 1300, and if you've been working with the thing for "months", it's pretty likely that you're over the limit.
The only other thing that comes to mind is that you might have SAVED the channels using one LNB type (LO FREQ), but then changed the LO FREQ, and when you go to the channel, the freq for that channel is out of range, however when you go back to the Antenna Setup page, the receiver tunes in your transponder, then when you go back to the channel, the settings are still wrong, but it uses what it was last set on. I have seen this happen on one of my receivers.
The strange thing though, is the several second delay after going to the Antenna setup page. This does make it sound like when changing channels, your motor isn't getting commands to go to the proper satellite, but when you go to the Antenna page, the dish DOES go to the proper satellite.
Again, I would recommend getting the channel editor program for your receiver (again if it's an ultra or another Fortec, it's usually a program called Gtools, but for other receivers, whatever they have), and check the channel data to see if there is anything unusual for the channels that are giving you trouble.
Perhaps a simpler thing to do would be to just delete and re-scan these channels, because it looks to me like the channel data is corrupted.