Time-based cryptanalysis: How to defeat it?

Assuming Alice is decrypting a secret message sent to her by Bob (on her very slow C64 ;), and Mallet is watching with a stopwatch in hand, hoping to determine Alice's secret key... It would be good to place inside the decryption routines a timer (WELL PLACED!) that waits a random-number of cycles (based on key-strokes, mouse position, etc.) to defeat this type of cryptanalysis?