Great chip circuit normally works first time, Intersil dual slopes are easy to use and top class data sheets, app notes!
Only as usual make a separate jig for simple test to weed out the sub standard stuff. Ground INLO and short INHI to REFHI. The reading should be near half scale in this case 10,000 +/- a few counts. You can improve this test by reversing the INHI and INLO signals, which also checks out another imp parameter, roll over error, the reading should be nearly same only with the -ve sign on. Practically any DG series or the ADG(Analog Devices)mux should give better performance than 4051.