Speedo Problems

[nf319]

i recently bought a chim 400, 1994 on a 'L' plate, very pleased with it, constant grin from ear to ear. i do have a problem in that the speedo gets to 78-80 and stops, i know i am going faster than that but it just wont go any further. When i slow down it tales it a while to 'release' and go down as well, i know the needle is free and will go past the 80 mark but not when driving. any ideas????