I had a new furnace installed in Oct and now its May and Im trying to turn my air conditioner on. The fan outside keeps turning on and off every 2 seconds. Air conditoner ( Fan) will not stay on. The thermost you can hear tuning on and off.

Is it just the fan? Does the compressor also cut off and on?

I'd have the installer come out. The line may not have been evacuated properly and has non-condenseable (water) in it and turning off under high pressure. It may also be low on refrigerant which means a leak. Leaks have to be found and fixed.

If the previous system was R22 and the new system is R410, the lines may have not been flushed properly.

Sorry,, I understood he had a new furnace installed not a new ac

Helo yes wmproop is right... I understood he had a new furnace installed not a new air ac (http://www.houstonairconditioners.net/air_conditioning_replacements.html)