Your looking in the wrong area.  Check your cards for cold solder joints.   
Symptoms you describe are indicative of this....  "leave it switched off for 30 min or more and turn it back on for some 5-10 min it hashes normally until the errors come again. "
Your ciicuit simply Heats up then errors begin because of the added resistance from the cold solder joint.

Cold solder joint looks Dull compared to a normal solder joint.  Sort of like a stainless steel mirror looks compared to a glass mirror.  (best analogy I could think of)

Anyway If your not sure what you are looking at, take a solder iron and touch\heat up each and every joint so they just melt and then solidify again. (They should look shiney) This will likely fix your problem.
Do not add additional solder and be careful to not burn the board. Have the board laying flat when you do this.  A 25 watt solder iron should work for you;  one of those big soldering Guns might be overkill.