1. Duration. What is the total duration of all the clips that you want to burn to one disc?
2. Capacity. Is your DVD disc single layer (4.7 GB) or dual layer (8.5 GB)?
These two relate to bitrate, as it is somewhat flexible how much a disc will hold, but it can sometimes be pushed too far, resulting in artefacts such as pixelation.
3. Types of pixelation. Some types of pixelation give visual clues to the cause. A disc reading error or signal/cable error may look different from a not-enough-bitrate error. Can you describe the pixelation that you see with your disc?
4. Saving as Disc Image (instead of burning to disc). [See also page 52 of the Toast user guide.] The resulting disc image file can be ‘mounted’ as a virtual DVD, and played by a software DVD player. Doing so will enable you to distinguish between encoding faults (the data) and disc faults (the physical carrier of said data). This is important as solving one does nothing for the other option. The disc image strategy this will also save a possibly wasted disc.
Set up your DVD as you normally would, but don’t click Burn yet. Instead, from the File menu, select Save as Disc Image. Select where the file will be saved. Toast will start encoding and then saving all of the DVD into a single file, <My_Movie>.toast . That file is the disc image, similar to .dmg files. Right-click (or Control-click) the disc image file, then select Open with > DiskImageMounter. Then it will appear on the desktop as a virtual disc. To play the virtual disc, simply open Apple DVD Player: it will look for mounted DVD-Video discs and play it. That player is quite strict, which is good for testing: if it gives issues, then there’s probably something wrong with the disc. If you don’t see artefacts (pixelation or otherwise), then the video file itself is likely fine, and it is time to suspect the media (the physical disc).
5. If the issue is with the encoding, and your settings are already optimal for quality, then it is difficult to diagnose without access to a problem-giving clip.