Coolant issue

I keep losing coolant in my vehicle. My temperature gauge never moves right of center but occasionally I will see steam coming from under the hood. The radiator was replaced about three years ago due to a crack. The reservoir is frequently dry but the radiator always appears full. This evening I added 50/50 coolant to reservoir and noticed pink "stuff" floating on top. I removed the reservoir and dumped it and noticed the bottom of reservoir has stuff that looks like it needs to be cleaned out but water has not done the job. I am letting water sit to hopefully soak off but what should I do now? I do not believe the reservoir has a leak so why is it going dry all the time. I have kept cardboard under car in the garage and occasionally a few drips appear from the same area as reservoir. Currently I am just driving it to work and back (about six miles each way) until I figure out what is wrong. BTW, my engine light comes on intermittently as well.