Stop using Apartame, stop using Splenda. I experienced the same symptoms as you, and with heavy use, I even had an occasional retrograde ejaculation, due to being so clogged with the junk. I now have zero problems since I went back to Sweet and Low. I'm telling you, this is the cure.
One more thing I forgot to mention is the fact that if it has been a few weeks without ejaculating the clumps find their way out on their own will urination.
Anyone ever heard of such a thing?