A wandering baseline in a large range slowly over an hour might related to pressure change such as small leak from the precolumn or another source.  Other possible causes include mobile phase that is not well-mixed (ex. topping off an existing bottle of mobile phase instead of preparing a new bottle, not giving time for new mobile phase to run through system).  Also, a drifting baseline could be due to changes in temperature.  Please ensure the HTEC door is closed, check the temperature setting and reading.