My timer slips the last 30 seconds?

When I set the timer say to 2 1/2 minuets, the thing winds down a treat but when it reaches the point on the dial that would conform to the last 30 seconds, it just sort of quickly peters out. It runs the last 30 secs in about 2.
Is this normal? I bought the toaster second hand and its been like this since it arrived. I'm an engineer and this failure to run the full distance has always bugged me.