Re-name your downloaded file and try again also look for the earlier Wheezy image the Beta.
The fact that you sometimes get the dialogue does not rule out a power supply issue although it is less likely.
There is some stuff aimed at noob support in the wiki the following link should help
http://elinux.org/RPi_Noob_Guides_for_W ... 16-18_beta
Consider the blind login with your RPi connected only to power and ethernet (no screen).
I have tried to keep these wiki pages as noob friendly as I can to the point where they may seem to be patronising.
Noob is not derogatory the noob is just the lower end of the noob--geek spectrum being a noob is just your first step towards being an uber-geek
If you find a solution please post it in the wiki the forum dies too quick