New stat...same problem

Hey guys, I posted a while back about an overheating problem. My 89 Suburban was purchased in the spring. It warms and idles fine but at speeds of 40mph+, even while cruising, it will jump in temp from 180-190 to 260+. It stays maxed for a few secs then returns to normal operating temp. It'll stay at that temp for a few secs then jump all the way up again. This temp jump stops at red lights etc. and the engine idles just fine. I have overflow of coolent, but just a little at a time. I changed the stat to a Stant 190 degree and have the same issues. Now, I noticed some crud, like a twist tie, for example, in the coolant resevoir when I added coolant the first time. Could something be pluging the water pump or radiator? And how the f*** would someone get a twist-tie ing with the coolant.