First of all, holding back on burning at the drives "optimal" speed is un-neccesary these days. In fact many people report that the burns are actually more reliable if you burn at the highest speed the drive allows.
However..... this is all well if your system is capable of shifting that amount of data to the DVD drive. So if you have a decent CPU, memory and HDD space (defragmented) then I'd suggest trying a faster burn speed.
The "dots" on your burnt media sound like buffer under-run problems to me, which shouldn't happen with todays drives.
What system spec are you using?
On a sidenote, I have personally had bad experiences with Arita branded disks. If you can get a hold of Titanium (Datawrite) media, you'll more than likely have more success.