First, which you probably already tried but just in case, work the cable as best you can to make it fit where you want it to go without any leftover twisting or bending torque at either end. Also at the same time see if components can be situated a little different as sometimes this alone will solve the problem.
Next step which a guy with this much already invested probably has done already is upgrade the outlet and since you know this is an issue ask about it as some do have a lot tighter fit than others. Or pull the receptacle, some of them once they are out of the wall you can get a screwdriver in there and bend the contacts a little to make for a much tighter fit.
The iec at the conditioner end at least you can shim it to fit tighter. Trim some thin wood shims, or wrap the end with something like teflon tape, whatever will make for a nice snug fit.
Next step which a guy with this much already invested probably has done already is upgrade the outlet and since you know this is an issue ask about it as some do have a lot tighter fit than others. Or pull the receptacle, some of them once they are out of the wall you can get a screwdriver in there and bend the contacts a little to make for a much tighter fit.
The iec at the conditioner end at least you can shim it to fit tighter. Trim some thin wood shims, or wrap the end with something like teflon tape, whatever will make for a nice snug fit.