HUD works when its hot??

[lilskyline240]

ok so my HUD has never really worked much ever...i swapped in a new cluster and the same thing...nothing changed....now i notice that the Speedo will only work when i have the car closed up and its been in a parking lot all day....then it will work right when i start the car up and will stay on till i turn the car off and have it off for awhile.....any clues?? i dont think the cluster is faulty...i think its the wiring....what could be happening??