Strange problem this, it may be that the unit (which unit is it?) is filling up with data more quickly than you would like it to (we should be able to work around it if this is the problem).
In the Settings | Options
menu, click on Advanced
and let me know what the 'Maximum Samples per Scope trace
' is set at ... tell me the timebase per division you are using and whether the LED goes out when the glitch occurs. Send a PSD file of the glitch to firstname.lastname@example.org
if its visible.
Also if you have the voltage range set to 'Auto
', try setting it to a fixed value, then listen to see if the relay still clicks every 3 seconds.
What parallel port mode do you have set in the BIOS?
Let me know how you get on with this.