Automatic and manual clock managing. Errors with ring counter but not with 4-bit counter
I think you have issue with signal quality but you need oscilloscope to find that. However it's hard to see from the schematic how the 744017 is connected and what exactly is the chip - are there any letters in the name like HC and HCT (they are totally different!)?.

Few advices:


Connect Clock directly to the AND2 input (i.e. isolate entire man/auto part) and see what happens.
If this helps - the problem is with 'Automatic Clock Manager' part. Check joints / replace the chips.
If this doesn't help - either you are coupling incorrectly TTL and CMOS chips (are you using HC or HCT chip version?) either you have some noise on the signal lines and you need filtering capacitors (but you can see this only with oscilloscope).


Hope this helps. If 2 doesn't help and you fall in 3 - just provide more details on rest of the schematic. Read also this https://www.fairchildsemi.com/application-notes/AN/AN-368.pdf how to connect TTL to HC. (HCT input is directly compatible to the TTL output).