Boxer cross digestive issues

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My parents' dog, Treacle, is a ten year old boxer cross and as she's got older, she's started having a lot of problems with her digestive system. She's always been a nervous type, not all the time but she gets too excited sometimes and she also seems to have a kind of social anxiety where it all becomes a bit much for her when all the family are around. That's not her day to day life though, usually she's just with my parents or one other family member at a time, and psychologically she's fine with that. Over the last few years she's started to be sick quite often, she's off her food quite a bit, and she has sloppy poos more often than not. She's had tests for absolutely everything and they've all come back clear, so we're at a bit of a loss now. She's happy and she's technically healthy, but she's still being sick sometimes and her poo is rarely normal. They've followed all the vet's advice and they've tried different foods and nothing seems to help. Can anyone recommend anything or suggest what the problem might be? Bearing in mind that we've been dealing with this for years so we've tried all the obvious.