2009 Hyundai Sonata, 164000 miles, automatic, p0462 code "pending"

Have the above car..three years ago mechanic replaced fuel pump and sending unit since car ran out of gas while fuel guage stuck on half.  replacing the sending unit kind of helped the fuel guage, but still wasn't reading correctly...it would stick on full then drop a quarter of a tank at a time.  I have used the car going off mileage for the past 3 years.  Just recently, guage would stick on empty, put a 'whack' on the dash above the cluster would cause the guage to go to full, regardless of how much gas in tank, so it has gotten worse over the years.  Decided to have it fixed.  Local mechanic changed the sending unit, did not fix.  Claimed to look at the wiring, said all good.  Decided the cluster was bad, purchased a used one with 160000 miles on it, put it in place and worked...for a week.  Driving down the road 6 days after picking up, needle went to empty again, drove about 5 miles, 'whacked the dash' again and it went to full with only about half a tank, guage slowly dropped back to half a tank (what is estimated to be in it based on mileage) as I drove it back to mechanic.  Pulled out the OBD scanner and came up with the p0462 "pending" but it hadn't tripped the check engine light yet.  It had previously been on prior to changing out the cluster.  Any help would be appreciated, but is this a wiring or harness issue, another bad cluster?  Something else?  Thanks