This is most likely your trackpad. Since you replaced your board and HDD cable there is not much left. Before replacing your trackpad, download Macs Fan Control and see what your battery temp sensor is reading. Normal is anywhere from room temp and up but nothing over 50. Your looking for some crazy reading like -144 or something of that sort. Your trackpad has a temp sensor and is a fairly common issue.
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=== Update (09/08/2017) ===
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Also check your HDD temp in the same program. Again... Look for crazy temp values.
This is most likely your trackpad. Since you replaced your board and HDD cable there is not much left. Before replacing your trackpad, download Macs Fan Control and see what your battery temp sensor is reading. Normal is anywhere from room temp and up but nothing over 50. Your looking for some crazy reading like -144 or something of that sort. Your trackpad has a temp sensor and is a fairly common issue.