Wireless keyboard right shift don't work with some keys but left shift works

So, this is really a strange behaviour under Mavericks 10.9.5

My right shift doesn't like some "keys" and some keys don't like the shifts ( either left or right ). For example:

doing a " ? " or a " B " or a " % " with the right shift doesn't work. I mean, after i remove the batteries and stay without using the keyboard for a day or two, when i get back to it, it works for a couple of minutes. But after that... i just can do a capital B or a ? with the right ****. Although i can do all other characters.

the " t " key is another one. No caps here, with either left or right shift key. But the key by itself is working just fine.

So i'm not really 100% convinced it's an hardware fault, because the all keys are working properly in other situations.

Any idea about this strange behaviour?

thank you for your time,

Regards

Pedro.