1989 240SX having problems with digital speedometer

[PimpJake420]

Just got my 240sx and its a real sweet car stock except the head unit in it but I've been having problems with my speedometer. On a nice sunny day when the suns beating down on the car the speedometer comes up right away as soon as the car starts but when its colder out and no sun (like at night or the early morning) my speedometer will kick on 5-10 minutes after the cars been running. This morning was weird though it never kicked on at all for a 30 minute drive. Anyone had any similar problems and know the solution on this?

Thank you I appreciate any help you could give me with this because its confusing me and my mechanic he cant seem to figure it out either.