Be certain the cross dove on top of the carriage has not become worn such that it is hour-glass shaped - narrow in the middle, or wedge shaped - narrow at front or back. You can measure this with a pair of small precision steel balls and a micrometer. The balls should be smaller than the total height of the dove but large enough to leave them protruding out from under the top edge of the dove on both sides. Set the balls on opposite sides of the dove, use a square to assure they are exactly opposite each other, them measure the width with a micrometer. (a bit of heavy oil can be used to help keep them from rolling away while doing this!) Any deviation in width indicates wear. This can be corrected on a vertical mill with a 60¡ã cutter or rotary hone to shave just a tiny bit off one or both sides of the dove, just enough to eliminate the measured anomaly. Mounting the carriage on the mill table in the correct alignment to make these cuts is critical. If you do not know how to do this take the carriage to a professional machinist.? Conceivably one can do this with a small hone by hand if it has the proper shape. Lay a strip of file card on the horizontal surface, you want to hone only the sloped undersides of the dove.
Another item is cross slide dove alignment. To check that lock the carriage at a position close to the headstock with a known flat plate mounted on the spindle. Set an indicator on the cross slide such that it's plunger contacts the plate surface nearest the operator and slowly rotate the plate to assure it is in fact flat- there should be no movement of the indicator dial. Now crank the cross-slide to move the indicator towards the back of the lathe, again the indicator should show no movement. The cross slide should be snug on the carriage dovetail without any twist or play at any point in it's travel - correct that first - see the discussion above. Then any movement detected as the indicator is moved from front to back shows the carriage dove is not square to the spindle axis. There is a gib between the carriage and the back edge of the rear way. If you loosen all the gib screws you can "twist" the carriage on the ways. Snug the screws just enough to remove all play then check again. If the indicator shows the cross slide moving closer or farther from the plate as it moves from front to back the dove is not square to the spindle axis and the ways. In a pinch you can adjust these screws so the carriage dove is square to the plate on the spindle, but this will cause the front vertical surface of the carriage to wear unevenly. Either the contact is at the headstock end at the the tailstock end. This will also cause uneven wear on that vertical surface on the front way and can cause it to become concave from HS to TS end. You may be able to get a small Arkansas stone in the "groove" in the carriage to hone the vertical surface in the carriage if correction is necessary.
Raymond
