If your monitor features both paralleogram and trapezoid adjustments (usually only found on "digital" monitors), you can sometimes use those to fix this at the expense of a slight pincushion. Use paralleogram to skew the whole monitor one way, fix it on one side with trapezoid, then use pincushion and pinbalance to fix up the other side. The resulting edges won't be quite as straight, but it can help with the squish effect. You can restore the appearance of straight edges by overscanning the video ever so slightly.
qrz: any idea what the cause of this might be? I'd have to guess high ESR or poor tempco leading to reduction in capacitance as DC voltage changes somewhere in the horizontal section, but I'm far from a monitor guru. I'd just like to know how to fix it. The Kortek/Betsons are famous for this, but it does appear to vary with what lot you get, suggesting it may just be a poor component substitution.