Do you have an Asio driver available for that 1082 ? If so, use that option.
Did Sonar set the 2048 sample rate during hardware acquisition or is that a recommended sample rate by Tascam? Try halving it.
The 1082 is firewire, yes? Some people get better firewire results using Texas Instruments based chipsets on pci firewire cards rather than relying on the MB firewire hardware.
Is this a new problem, or has it never worked correctly?