04Silverado

Ok, so yesterday I was driving my 91 c1500 and all of a sudden the check gauges light pops up and I also noticed that my temperature gauge was almost at 260, then while I was still driving, it when down to just under 210, where it usually is. I also noticed that when it goes down to normal temperature, if you accelerate hard it goes back up sometimes almost to the red but in a few minutes goes down to normal. I got home, let it cool down and checked the coolant and it was pretty low but I couldn't find any leaks. Just hoping the leak isn't internal. Has Anybody had this problem before please chime in and help me solve it. Thanks in advance.