my tach and speedo have jumped up and down since i got it. i don't really think any of the gauges should be taken as any more than just a suggestion.
i would check your 02 sensor and your cap and rotor. if the tach only jumps while it's doing the chugging, i would lean towards spark leak. on your cap and rotor, check for cracks, or excessive dust or wear inside the cap, or on the rotor. if there's a hairline crack, it should be visible with white scorch marks. the plug wires you are going to put on may also help.
why have you put four motors in in 300000 miles? i'm still on my first - what the hell are you doing, burning diesel in them??
need to figure out why check engine light is on. if its an o2 sensor can mess with timing advance and cuase timing issues while steady driving and not while under any sort of load