Check voltage on pins 1 and 7 on all of the opamps with power on. They should all be at or near 4.5V. If they are, then the opamps are good and there is no point swapping them out. Those cheap stamped IC metal sockets will get loose and make poor contact if you cycle them too any times. All of the parts I checked looked correct. R27 is strange in the BOM & schematic. a 1Meg resistor there makes no sense. Wouldn't make it do what you're experiencing, but it is strange.