I've had something similar happen to me and the longer you try and ignore it and think it will go away the worse it gets.
Try speaking to them without your spouse there and ask them why you are hearing these bad stories about your spouse and why you are being told they are coming from them. Try and listen and be open and find out what the real issue is. Once you find out what the issue is you can take the steps required to deal with it.
If you discover it's really a non issue or something that makes no sense logically etc then try and explain that it is putting unnecessary strain on your relationship and that it would be best if they brought their issues to you in the future and not to discuss it with others. Either way, I would still stress that last sentence to them as it's really no one else's business....
Sometimes when a son in law helps his in-laws they feel embarrassed and this is how they deal with that embarrassment, by belittling and undermining the person who helped them. I know it doesn't make sense logically but it's really common. As I said, I have been in this situation and have seen others in it too. Next time, either an actual relative should help them and you should tell them that you are really sorry but your partner is really busy and can't help.