First off, dont use MME. Thats not a driver mode for audio recording when using a sound card/audio interface.
2nd, make sure you install the latest drivers for the lexicon.
3rd, make sure you have your lexicom set up correclty in sonar in the preference menu. IE: driver mode, recording and playback I/O timimg masters
4th, Then try WDM and ASIO driver modes to see what works best for you. In ASIO mode, you need to adjust the ASIo buffer for optimum performance. In WDM mode, you slide the slider to find the optimun performance.
Cj