tacho problems

[ccharlie6]

i've just fitted up my smiths RVI tacho up (the one with moveable redline) and upon starting up everything seemed fine, only for 5 seconds later the needle drops to the needle stop and then back to the correct revs and then continues doing this. Anyone any idea as to why its doing this?
its all calibrated correctly and the sender is connected onto the -ve side of coil

any help is good thanks in advance