2002 Mitsubishi Galant Car Overheating; replaced everything

2002 Mitsubishi Galant, 134,000 miles. My car is overheating, and has been for quite some time, despite replacing the thermostat numerous times, hoses, and the radiator. It all seemed to start when this red, rust-colored, paste-like deposit started to cake up in my reservoir tank. Once I noticed it there (several months ago), I also found these deposits in my radiator and hoses, hence the reason I replaced them. Thinking that replacing all these components would fix my overheating issue, well, it hasn't. I have no leaks at the hoses, radiator, or thermostat. I was told that these rust-like deposits can be due to putting water in the radiator, but all of my components are aluminum, which does not rust. I've also been that using the "red" coolant can do the same thing, if mixed with regular coolant. I've never used that type of coolant, unless my radiator was topped off with it by a mechanic when I got my oil changed. So, if all my components are aluminum, then where did these rust-like deposits come from, and how do I clean out my engine of this stuff? I've used 2 fluch kits, with no luck. I need to mention that the red deposits are not huge clumps, it just covers my hoses and thermostat with a thin film. I suspected that it was clogging up my thermostat, but then again, right after replacing, it still overheats. I'm lost! Is the next suspect my water pump, even if it's not leaking? Could it be clogged with these deposits as well, causing it not to pump properly? Have you ever heard of anything like this? The mechanics I've spoken with are all quite perplexed. Help me, please! Lol.