My Taxi's spinout was doing same as yours only getting about 50,000 every time. I found that this is directly related to leveling. I put a new tip and spring on the shooter rod and it made little differences. Then I spent $35 for a craftsman level (you can read about that level in a thread I started) after I perfectly leveled the game at an exact 6 deg and level side to side. I am getting 100,000 very regularly with an occasional 75,000. If you over do it you'll end up going past 100,000 and getting the 1k bonus.

Do you find that your ball after it goes throughout the spin out and the kicker kicks it to the top, draining a lot down the Gorbie/Jackpot orbit, instead of one of the 3 lanes? If so you may have a slight playfield twisting going on. I know on mine I was having a hard time getting both the bottom (near the slings) and the top (near the lanes) to both be level at the same time. And as LTG said, it turned out that my left playfield hinge (the one below the left sling that connects to the bar lock down area) was missing a screw and the other was loose. After I added a new screw and tightened the rest up I was then able to get both the top and bottom level and at 6 deg.

Thought you already did replace the rod you called it the plunger? Did you mean the rubber tip?