An easy way to determine if overcarbonation is your problem is to pour a pint, stir it until you knock most of the CO2 out of solution and then taste it. If the off flavor goes away, you overcarbed which is an easy fix. Take it off the gas, activate the relief valve if your keg has one or if not press the gas in post until it stops hissing. Let it sit for a day at room temperature. Hook up the liquid only, pull a pint and see if it's better. If not, repeat the whole process and keep trying day by day until it's no longer overcarbonated.
If overcarbonation is not the problem, then start looking at replacing equipment. It's easy to overcarbonate beer by setting the regulator too high and leaving it there for several days too long. The probability you overcarbonated by accident is far more likely than bad/old CO2 or a bad tank.
