that don't sound right. lightly hold the brake in D and see if the car actually surges forward. if it does it might be a vacuum leak. if the car is mis-firing due to bad plugs, wires, cap, or rotor it will do the same thing. too an untrained ear you will never know the car is mis-firing. to a trained ear you can actually count the cycles and hear it even if it is very minute.
you won't notice a power difference that much in 1st and 2nd if it's an automatic car but when you hit 3rd it will flatten out and actually the rpm's will fall back as if your spinning in 1st gear and the car catches gription and makes the same sound in 3rd gear as the motor tugs down due to not pulling from the torque.
check the wires/plugs/cap/rotor if that checks out fine get some carb cleaner and start spraying around vacuum lines and listen for the motor to tug down a few rpm. something is not right i couldn't see the tach start getting all freaky like that for no reason. i would think it would just stop working.
i was racing my friends LS1 TA that has 340 rwhp and 340 rwtq and i burnt 2 plug wires to the core that night playing with my friends one night. on the highway when i hit 3rd gear at 90 mph the car just fell on it's face and tugged down. from a dead dig i tore the LS1 TA up bad due to me having more torque until i hit 3rd gear but by then he was coming around me pretty good.
good luck and let us know what you find!
