c/c problem

[go fast]

my digital c/c has gone haywire,it comes on when it wants to,and when it comes on and you stop and turn the key off to go to the shops-ect it wont come back on until it decides to,no demister when it rains nothing at all,had the digital panel tested and that works fine,when it goes and you stop the car turn the key off go somewhere shopping or something turn the key till all the dash lights show up it comes on and when you turn the key the rest of the way to start the car it turns off,and comes on about half & hour to a hour till it decides to come on,its got my head spinning any body go any solutions to my problem?any ideas would be a great help.