Earthshaker is about the only pinball I can think of that I put a RED highest tension plunger spring available on.
Make sure you have a good plunger rubber tip.
The outside barrel spring is usually compromised on Earthshaker pinballs (small spring on outside of pinball by the plunger knob). The extreme power of the red spring usually compresses this, and it is frequently replaced with a spring that is too large... which keeps the plunger further from the ball in rest position. The standard part number for this spring is #10-149, but I will substitute a slightly shorter spring like the 266-5010-00, if I'm not getting close to the ball with the plunger tip in rest position.
It's tempting to just smooth the plunger rod when the chrome starts flaking, but just replace the plunger, and the plunger sleeve. Small amounts of drag cause big effects on the power of your plunger.
Weak plunger shots are almost always caused by the playfield 'sagging', and the plunger no longer being aligned with the ball.
Think of a pool cue ball and your cue stick. If you hit the cue ball toward the top of the ball instead of dead center, it won't have all the power that it could. Similarly if you hit the cue ball off center to the right or the left, it won't go straight away from you.
When you push your plunger in toward a ball it needs to be exactly in the middle of the ball. If it is to the left or the right, the playfield may have shifted right or left. Sometimes I'll install shims to reduce the tendency for the playfield to be 3/32 to the left or to the right.
Almost every pinball with a plunger this age will have the plunger hitting the TOP of the ball instead of dead center.
The fixes for this are drastic.
Remove your plunger assembly.
When I'm prepping a machine this age I will get a wood rasp, and the bottom of the slot and the bottom of the left and the right wood that holds the plunger in place... I will file away at least a 1/16 of an inch to allow the entire plunger assembly to drop down. Frequently I need 1/8, or 3/16 of filing to do this properly, and that starts becoming quite visible from the front of the machine.
So instead of doing all the work with filing, I'll file some, drop the plunger assembly down a little, and then use a shim on the outside bottom. For years I used a big thick washer, about 1/16 of an inch thick. Slide the washer between the cabinet and the bottom of the shooter assembly as you put the shooter assembly back on the game. Pinch it in place as you tighten down the screws.
This has the effect of 'tilting down' the angle of the plunger. This, coupled with a slight general lowering of the plunger assembly will generally get the plunger in line with the middle of the ball.
This will be slightly visible, the outside bottom of the plunger assembly won't be snug against the wood.
So, to sum up...
Pinballs are very parts intensive. To fix most things on pinballs you will need to replace parts.
If I had an Earthshaker to refurbish I would presume that I would need a Shooter Rod, Shooter Rod Sleeve, a couple of shooter rod washers (they are always missing), a RED plunger spring, an assortment of outside barrel springs, and a new shooter tip. I prefer the heavy rubber shooter tips from Suzo-Happ, though I buy very few other pinball parts from them.
I would presume that I was going to be removing the shooter rod assembly, and filing down the left and right bottom edges of the slot, as well as the bottom-most area of the slot, at least 1/16 of an inch, maybe more. I'd presume that I would be putting a washer in the bottom outside of the shooter rod assembly in order to line up the shooter rod assembly with the ball.
This would just be my standard thinking when approaching an Earthshaker.