What Causes White Clotty Discharge From Penis During Urination?

I m a 44 year old man. I drive school busses and vans for a living. I ve had prostatitis in the past. For the past month I ve been having trouble urinating. It burns and sometimes the flow stops mid stream. When that happens, I wind up squeezing, in a milking motion, my penis into some toilet paper, and there is always a white chunk that comes out and when it does, I can finish urinating. Today, this happened and a lot more than normal came out, but it was more yellowish than white and had the consistency of snot. I also noticed a small amount of blood in this discharge. My family doctor has not been able to figure it out, nor had the urologist that I was seeing. Do you have any ideas of what this could be?