Car rpms dropping quick

[jt98boost]

My 98 cobra will drop rpms very quick if I shift to neutral while cruising down the road, it will drop to about a 1000rpms surge up to 1400 then back down and die. I have replaced IAC, Throttle body, tps is set on 1.00v, dpfe sensor, I know lot of guys have the problem with the idle hanging between gear shifts, but my problem is the opposite, my rpms drop quick and If I don't go to another gear it will surge once or twice and then die can't hold a steady rpm. Anyone got any ideas????

 

[Kilgore Trout]

Is it driveable or is the problem too severe?

Could it be the fuel filter? It seems like it is fuel related...

 

[jt98boost]

I can drive it, just got to be carefull not to leave it in neutral while rolling, or it will surge a couple of times and then die. It ildes fine with the car not moving, but if you get going about 30+mph and put it in neutral it just will not stay at an rpm it will drop and surge then die. I changed the fuel filter about 6000miles ago, it has a new 255 vortech intank pump, a t-rex inline pump, 42lb injectors. The fuel pressure regulator is stock, the car has a vortech supercharger.

 

[jt98boost]

I have the bosch stock bypass valve, just wondering if that would cause the car to do this, it has been on the car since I purchased it 3 years ago thinking it may have went out?

 

[WillDuhhBeast]

Egr?

 

[jt98boost]

no, already have a new one

 

[WillDuhhBeast]

Is it throwing any codes?

Usually dropping rpm's has something to do with fuel or the vacuum.

ummm...

idk =x I'll :Zip2:

 

[jt98boost]

Yes, it has a p0500 speed sensor malfunction, but the speed sensor has been replaced. I can rev the car and at 2000 to 2200rpms it seems to shake/miss and then the check engine light will pop up that code ?