Periodically experiencing cross eyed effect when using both eyes

Hi,

About 6 months back I experienced a loss of focus(feels like looking cross eyed) while sitting in front of the computer. This is remedied by trying to focus on a different item/blinking or closing my eyes and then all goes fine for a period until it randomly happens again (no obvious pattern) and lasting between 1 and 5 seconds. My vision is fine when I look through only 1 eye (either one) while I am having these focus problems. This focus problem surprisingly does not happen while I am at the optometrist (reminds me of my car) and my doctor did not seem worried about this and said that it might be eye fatigue etc. I have since been on a month holiday (without computers) so I ruled that theory out and it is still happening and not so much fun if it is happening while driving....

I am a 29 year old male that is fairly active with normal blood sugar and blood pressure. I wear glasses most of the time with prescription -1.5 for both eyes and slight astimatism. I further suffer with sinus problems and treat it with 1+ cortisone injections a year and quite a few anti-histamines (such as texa and allergex) but I still sometimes experience blood in my sinuses and fairly regular headaches.

Is this something I should worry about and what type of doctor do you go to for something like this?
Thanks in advance.