Can't install Yosemite can't get back to Mavericks

Hoping someone can suggest something because I am at my wits end with this system.

Using Mavericks after a Time Machine Restore I have a current problem with Finder whereby if I click on a Shared Network drive it simply shuts down the finder window and restarts it... I can never see the shared drive when its in the sidebar...however when its in the main window of Finder I can sometimes navigate through it... and some apps can access it and others can't.. bizarre and inconstant except for the sidebar actions - i.e. shuts Finder Window and Opens a new one ...

So I upgraded to Yosemite a while ago and have noticed almost daily crashes ..... eventually I got fed up and thought I'll go back to Mavericks ..it was more stable..

So I jump into Recovery mode and try to reinstall the OS that came with the Mac but i clearly made a mistake because that ONLY wants to install Yosemite now. So I think wth I will try a clean install of Yosemite but no that refuses to install .. I get the usual messages about corrupt downloads etc and i have tried very combination of setting the date suggested everywhere else....

I created a bootable USB Yosemite install and no it won't load either ....gets about 30% through... then just restarts and goes back to the recovery mode prompt...

Bear in mind I did this maybe a dozen times and no luck...

The ONLY way to get back to mavericks is to reinstall from Time Machine BUT that gives me the problem I mention above ....

I have even erased the **** drive hoping it might allow me to reinstall Mavericks but no joy at all... it always wants to install a clean copy of Yosemite and that fails every time.

Is there no way to get Mavericks install image back to put this back to factory default state? Everything I see says Apple don't let you download it if you downloaded Yosemite.

Going to shoot this machine I think...at the end of my tether..