Have your freon evacuated and recharged. Make sure you you get a competent tech.
When I do evac & recharge, I write down the system specs. If its given in Lbs, I convert to Oz. then evac and see what the machine pulls out. Then compare that to a full charge.
For example:
Full charge: 28 oz
Recovered: 17 oz
System low: 11 oz
Then I look for a leak, or charge it with dye.
Either way, more than likely you've never had your ac system checked. Over time, you can lose some freon, even though its SUPPOSED to be a closed system. When you lose enough, then it starts acting exactly like you've described.
Don't try and buy a freon guage from the parts store. Only a evac and recharge will get your system to the proper specs.
Hope that helps.
Auto tech for 16 years.