I know that hind sight is a wonderful thing, but designing in EMC immunity from the start is the only way to go. Trying to retrofit a solution can be difficult and expensive, this is, or was the first thing they teach you in EMC classes.
You don’t actually say what is causing your problem, this would help in giving you some quick fix advice.
Everything Klaus mentioned is all sound advice. Being perhaps a little more detailed for high frequency immunity and assuming that it is an op amp input or low voltage digital input that is giving the trouble try putting a series resistor close to the input and perhaps a capacitor from input to ground or whatever the input may refer to as ground. As to values that will be circuit dependent, again making the assumption that fast response is not needed for a 350MHz interferer 1nF would be more than enough for the capacitor, 100 to 1kohm should do for the resistor.
If you have any long leads connected to your circuit this will be a good way for the signal to get into the unit, a 25cm lead length makes a very good antenna at 350MHz.
All this is quick fix stuff, if you want to place your unit on the market it really needs designing for a hostile EMC environment, even if your local regulation do not require it. The product and your company will soon get a reputation for being unreliable, and depending on the application dangerous.