I pour in the priming sugar solution while it is still quite hot after I’ve run about two quarts of beer into the bottling bucket. I’ll use at least 16oz h2o, on the thin side- this ensures the solution incorporates evenly and well. If you are waiting for it to cool and it is thick you’ll run into problems with the mixing.
Sanitation! Make sure your rinse your bottles well as you use them- store them upside down so all the water drains out and nothing can drop in like dust or whatnot or grow in there. I use the dishwasher on sanitize setting and bottle off the dishwasher. No mess.
But I have used starsan or iodophor and it works well. Baking bottles was where I saw breakage and infections.
+1 to everything Euge said plus a couple more recommendations. I use a sanitized spoon to gently stir the priming solution/beer mixture in the bottling bucket a couple of times while racking. I also leave my tubing a bit long and let it curl around the inside of the bottling bucket. This way when I rack over the beer swirls into the bucket and helps it mix with the priming sugar.
To clean the bottles, I usually run them in the dishwasher (no detergent) right before I’m getting ready to bottle. I open the dishwasher for the first time as I start bottling. Is this not good enough?
I’m assuming I do have a sanitation problem, if that’s the common thread here.
It’s probably more likely an infection, but inconsistently mixed priming solution is an easy thing to fix. I sanitize a long spoon and give the bottling bucket a few stirs every 6 beers or so.
I know we talk about changing one variable at a time, but in this case I’d tackle both. Rinse all of your bottles right after you empty them, do the dishwasher thing as long as it gets hot enough (dunk in sanitzer if not), and make sure the sugar is well mixed in the beer when bottling.