Topfield 5800/Stable Toppy

Chunkywizard's Guide to a Stable Toppy

Load on the recommended firmware (currently 5.13.65). Although this is fairly stable it can be improved by patching it. There are a number of bug fixes; I would recommend the following patches as a minimum: 1365BfBqCeCyDeEcErEsFGmHMhNfPPcPfRReRpRsSSrStT2XpXw. A pre-built copy of this firmware can be download from here. Note if you are running EPG Navigator and UK Auto Scheduler you may want to drop the De patch as it is not supported by these TAPs.
Check that the channels are correct. They should be as per this channel list. Check you have no duplicate channels and no channel numbers in the 1000's. If you have then consider using the ChannelManager TAP or the newer Channel Organiser TAP to sort this out. Note that if you have used ChannelManager and used the option to renumber regional variants, you may have additional channels (typically in the 200+ range).
Check that your signal strength is acceptable. Try to get 100% quality across all muxes and above 60% strength. If you are unsure of your signal strength then use the SignalMonitor TAP or the manual Service Search in the Installation Menu (see here for details about how to do a manual scan, and then sit back and watch the real-time display — press the Red button on this screen you get a bigger display that beeps a lot).
If you are using TAPs then make sure they are all up to date (using the latest stable release), use the default configurations where possible, and try only to use mainstream, UK supported ones (so help can be provided on the UK Forums if issues occur). Fewer are better, in my opinion.
Interactive Services (MHEG)
The MHEG engine (used for the "red button" Interactive Services) on the Toppy is known for being of dubious quality. While 5.13.65 has the best implementation of this, it is still an area of concern. If you want to be extra careful consider turning off MHEG in the main menu. If you wish to use it (for radio for example) but wish to protect yourself then consider a TAP like MHEGControl.
Some Light Reading
Check the manual for the Toppy (and for the individual TAPs you use), and consult (and search) the UK Forums for any issues for the setup you have. Some interactions are well known and documented, and this will save you a lot of time and swearing. Also, browse through the Troubleshooter for some help on common problems.