If you have a slimline 5 dish installed (non-SWM) and you change the setup in your receiver to slimline 3, this will NOT solve the problem. The slimline 5 LNB uses the guide data from the 101 satellite when you are tuned to a channel from 99/101, and the guide data from 119 when you are tuned to a channel from 103/110/119. The slimline 3 LNB and all SWM LNBs use guide data from 101 whatever channel you are tuned to. This use of guide data is built-in to the electronics of the slimline 5 and slimline 3 LNBs and changing settings in the receiver won't change it. This means if you set your dish type in sat setup to slimline 3 when you actually have a slimline 5, you won't get any guide data when you are tuned to a channel from 103 (where most HD comes from) so if you leave the tuner on a 103 channel, you will eventually get a diagnostic message telling you you are not getting guide data, and you will start to see gaps in future guide info.
The solution is to replace the LNB with a slimline 3 LNB, or any SWM LNB.