Sporty Speedo Problem

Here is another issue with my 03 883. My speedo works fine all the time, however if I stop to fuel up, when I start up the speedo reads slow. If I then stop, turn the ignition off, re-start, the speedo reads true again. If I stop for fuel and turn the ignition key to acc only to turn the engine off but keep the speedo powered up, it is ok. If I fuel up then cycle the ignition twice before starting, the speedo is ok. I always let it boot up in acc before turning the key to run. Also if the bike sits for say 10 minutes before starting the speedo reads true. It is just the quick gas up stops that affect it. I have changed the speedo head and cleaned the speed sensor. I have checked all the connections etc. Ideas??
Cheers, Scott