Default [Q] Full memory on android 2.3.6 cause not cache, Dalvik cache or install app

My device running android 2.3.6, mtk6573 chip, after using at some hour a memory at full alert. Free about 150mb, decrease down about 1-2mb, after system alert and free 150 again. Despite clearing cache and wipe Dalvik cache but not effective. I was looking on google and xda but no instructions. Please help me to overcome.
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