1992 Dodge Grand Caravan Speedo problem

We have a 1992 DGC that has an annoying speedo problem.
This only happens on the 1st start of the day, you start the van, start to drive and the speedo is dead.. get up to 50-60km/h and BANG the speedo springs to life!

Has anyone got any clue what can be casuing this?

If it is the speedo sensor on the tranny, can you tell me where it is and the easiest way to change it?

Thanks, Gordon