First, how you set the TC to be a base station? The questions in the setup very confusing.
Next, is it everytime need to use airport utility to change any of my setup? My problem starts when I start changing settings on MB and PC. I think it is what you suggest change to DHCP to solve the issue.
internet -> modem-wired->TC-wired->PC (PC and TC be on same IP range)
TC- wireless-> rest (same ip range)
Set static ip for your modem/tc first, then your PC. The rest use DHCP to auto assign for you. Specify the range available when you set up your TC first.