The tachometer in my car does not read correctly when the car has been in the shade but when the car has been in the sun the tacho reads fine. When I started it this morning the tacho didn't move from zero for about a minute or so but the the car was idling fine when this was happening. What could the problem be? cheers.

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It could be the 2 capacitors on the back of the tacho itself. It's quite common for them to go bad (mine did). 
It's easy to replace them. If you do a quick Google search you should find plenty of related results.