The missing letters thing might be an application bug, and not necessarily OS or computer dependent. It shouldn't drop letters no matter how fast you type. Just out of curiosity, if you just open up Notepad and type into that, can you type up to your full speed?

Another thing it might be is a bug in the keyboard or keyboard firmware, such that it cannot detect a letter when certain pairs of letters are pressed at the same time. Do you find it's only with certain letter combinations that it drops?