External Display through VGA connector works on 720p but not on 1080p. Worked once after SafeMode but not anymore. Please help!

Tried the following:

1. NVRAM cleared

2. Safemode

After safemode it worked perfectly fine on 1080p but suddenly Samsung Monitor's screen went blank again.

It works if scaled to 1280x720.

Doesn't work on any other resolution either.

How can it work once and not anymore?

Few months ago also I tried the same thing happened - worked once for few mins and again blank.

Please help. I really need this working.

Macbook Air Mid 2013

Yosemite 10.10.4