Tach, abs, tcs and misfiring all at once
 

This is on a 98 C5.
The car was running great and everything was working besides an exhaust leak. I dropped the driver side header, bolted it back up, changed the oil and the spark plugs. After that the car wouldnt start, it wasnt getting fuel so i put in a new fuel pump. The new pump sometimes works sometimes doesnt, when it does itll run full blast the enitre time the key is on.
Also my abs and tcs lights are on and the computer is always chiming about it. Then my tachometer will either stick @4k rpms and bounce no matter if its idling or driving or it will simply sit at zero and not move at all.
While driving the car will misfire pretty badly on acceleration but its still drivable and never stalls.

What is tied in w/ all of that because I have no real idea where to look. Ive looked over it alot and cant find anything, unhooked the battery 3 times, checked fuses and relays swapping them around and nothings changed. Help please.

Sounds like an electrical issue. If its not a battery connection its possibly a loose ground or positive connection somewhere along the chain. Could a wire of been frayed while dropping the header. Double check the charge on the battery, and alternator output.

Im going to add that i swapped from the nitrous plugs back to the regualr motor plugs and the misfiring is gone. For some reason the car hates the nitrous plugs.

What really gets me is how the fuel pump ony comes on sometimes. Usually when the car is cold its come right on and if the car is warmed up it has to sit awhile. Does the crank sensro control that and the tach?