Run the diags on the hard drive to make sure it's OK. 

 

Unfortunately, it may take a complete "clean" install to fix the problems.  e.g. reformat the hard drive and reinstall everything.  I know it's a hassle but sometimes that is what is needed. 

Whoa that sounds scary, but it sounds like a possible cause. Could you advise more how to "run the diags on the hard disc"?

Thanks

There are several options, depending on what you have.  There is a Dell diagnostic partition on many new Dell's and that allows you to run hardware tests. 

 

Otherwise, you can run error checking and discdefrag on the hard drive.  If there is a bad spot or corruption it should show up. 

 

Open the Windows Explorer, RIGHT  click on the hard drive.  Then LEFT click on the Tools Tab.  You can run the "error checking" and "defrag" from that panel.