Stiletto,
Which version of software are you using?
There are a couple of things that can do this. First and most obvious is that the processor use will be related to the number of tags being monitored and the amount of historical data in the database. Is this a large db with lots of tags?
Second is a possible corrupted database. If there's some corruption in the database files, the historical data viewer will have to work harder to process the data. You can check this by changing the folder for the database and seeing if the problem follows the new database.
Let me know how it goes.