When writing in Sibelius 5 using the VDL2.5 template I find that during playback there are ";ticks";. Some times one line will note play very clearly like fumbling on a grace note or a diddle, but the next time I play it back it's fine. Some times one section of the measure playing has a tempo propblem slowing down or speeding up a little, like dragging or rushing. Why are there these ticks in playback?

This sounds like it may have something to do with your audio interface. What kind of soundcard and audio drivers are you using? Also, have you checked the latency settings in the Sibelius audio hardware options? (accessed via the playback devices window)

Depending on how much polyphony you're trying to churn out, it could also have something to do with the amount of memory you've allocated to DFD for disk streaming. Have you fiddled with any of those settings to see if it helps?

Sigma Tel Audio, STAC 92XX C-Major HD Audio. Under Audio Engine option in the play back window the interface is Primary Sound Driver (ds) maybe it should be Sigmatel? Buffer size 4096 sample rate 44100 HZ latency 92.88.

Raising or lowering buffer size will raise or lower the latency value. The optimal setting will differ on everyone's machine depending on system specs, and what sort of music you're producing. But you should probably start by carefully reading the FAQ about soundcards here:
https://www.tapspace.com/support/faq/index.php?sid=442262&lang=en&action=artikel&cat=4&id=33&artlang=en

By your description, you're probably using the audio interface that came built into your PC. While this will be fine for simple stuff like playing back MP3's, it's likely going to struggle when you're throwing a lot of processing at it. This is why upgrading to a soundcard with ASIO drivers is typically recommended for PC users as you may reach some quick limitations with DS or MME drivers on whatever Dell throws in as a stock soundcard.

If you want to fiddle, i'd start by lowering the buffer size (hence lowering the latency). Going too low will put excess strain on your CPU, but going too high (where you're at right now) will likely cause the rhythmic burps you're describing. I've found VDL and Sibelius produce good results with a buffer size of 1024 - but that's assuming your soundcard can handle that. Ultimately, you'll see much better performance with a soundcard that's more apt to handle pro audio with ASIO drivers.

You'll also find some information about these various Windows driver formats (ASIO, MME, DS) in the glossary of your VDL user guide.