Fractal Noise / Displacement map... loop STOPS after a few seconds of animation in After Effects

I am working on a collection of music videos.  I have created computer animated visual images in After Effects, like waves of water created with "fractal noise" and "displacement maps."  According to the tutorials I am watching, I use "expressions" like +time*150 or +time*200 to animate the fractal noise.  At first it looked like everything was working, but the fractile noise stopped moving after a few minues.  (The water literally FROZE about four minutes into my nine minute video.)  I tried increasing and decreasing the numbers, like +time*2000 and even +time*20000.  Nothing worked.  I deleted the expression and tried animating with key frames.  When that didn't work I deleted the key frasmes and went back to the original expression +time*200.  Finally I "rendered" the video in Premiere, thingking maybe it only appeaered to stop becuse I was watching a "preview."  One I finished rendering, now it only last 16 seconds?  Does anyone have any idea what I'm doing wrong?  Everything I try only seems to make it worse, and I'm doing exactly what the tutorials are telling me to do.  However, my videos are anywhere from nine to seventeen minutes long.  The "fake water" has to keep flowing for the entire video, and the lights of the city need to keep twinkling.  I'm almost done with the entire project.  The only thing holding me back are these displacement maps.