Bill
Using LM324 or LM358 op-amps are probably some of the worst choices to use for ADC voltage buffer/follower applications (uA741 is also a poor choice). The better op-amps would be rail-to-rail op-amps They give much better performance and far more stable in these applications.

Something like MCP6022 C-MOS OP-OP-AMP or any that can supply rail-to-rail on the input/output, Well I found them to work great