Sorry for the unmentionable subject, but I thought it might be useful to post the outcome of my self-research and experiment over the last few years. This is my experience. It may not help you, if you also have D. problems, but maybe will help you think about the possible causes.
When I first arrived in Kunming, around five years ago, I decided to try eating whatever and wherever my local Chinese friends did. I enjoyed a wide variety of food, but spent an inordinate amount of time in toilets and used up huge amounts of toilet paper!
Around three years ago, I thought I would start experimenting to see if I could identify the causes and try and reduce my visits to the loo. This is the (incomplete) summary of my findings. I stress it has not been possible to fully eliminate D. from my life, but almost...
1) Tap water. This is the biggest single enemy. Washing salads or other vegetables is guaranteed D. I use mineral water (MW) for all cooking, including boiling veg and even rice/noodles/pasta. I carefully protect my plate, knife, fork, spoon, toothbrush and wash in boiling MW before AND after use. I do not share these items with my family. Ever. I am careful in the shower, not to get TW in my mouth, and all other locations
TW is the enemy. Avoid!
2) It is normal to eat with your bare hands. Don't! I have found it is impossible to satisfactorily clean my hands, so safer never to touch food with my hands. Most places supply free plastic gloves, or you can buy your own very cheaply. NEVER touch the food!
3) Spice. Yunnan food is almost always spicy. I had no significant D. problems in Shanghai, for example. Eating only at home is the safest route, because almost all Yunnan restaurants use chilli spice in all of their cooking. If you ask for 'bu la', no spice, they simply think you mean no additional spice, and the food will still be spicy. My tolerance for chilli spice is completely zero. Most restaurants use the same oil and the same cookign equipment for all their cooking, spicy or non-spicy, so the food will almost always be contaminated by spice, even if they don't do it deliberately. You can try using the phrase guo min (allergic), or ling la= no spice, but they have no understanding of anyone living in Yunnan wanting absolutely no spice. I often say yuan zhi yuan wei = original flavour/taste, but they look at me like I am stupid. Best to avoid all Yunnan restaurants and cook at home. Sweet foods are usually okay, but the new chocolate dessert in BK is spicy! Probably because the deep fat frying oil is spicy and it is cooked in that.
Summary - there are almost no safe restaurants in Yunnan. McD, KFC and BK have mostly added spice to their standard products so are inedible. BK's beefburgers seem okay - double cheeseburger is my favourite. Pizza Hut also seem to have some original non-spicy pizzas, . The other option is the buffet restaurants, where you are given the raw meat and can BBQ it yourself. Then you can avoid the spice altogether, but beware the spicy hotpots as the smell may also cause D.
Best of luck!