How To Treat Painful White Chunks Coming Out While Urination With History Of Kidney Stones?

Hi for a few years now I have been having (off and on) these painful white chunks come out while urinating. Sometimes it clogs my ureathra but eventually they pass through. Very painful and my penis will usually have a burning sensation for an hour or so afterwards. Like I said this has been off and on for a few years now. I did pass a large kidney stone 2 years ago. (I actually had to go to the hospital due to it being stuck between my kidneys and bladder) but these are different. They will actually clump up at the tip of my penis to where I can actually get some of them out with the tip of my finger. They break up very easily when rubbed between 2 fingers. I ve been to the doctor a few times for this. Every time I go I m not having the problem by the time I can get an appointment so when I do the urine test it comes back clear. I know it s not an STD but it s very painful and annoying especially when in public situations. What is it?!?!