check engine light

[Red90SI]

Hey guys i have a problem when i run the car hard for a min. i get a check engine light but when im stopped if i shut the car off n start it back up the engine light goes out not shure if im really messing up my motor n its not telling me of if its a glitch in the system somewhere. i have a 1990 civic SI with a jdm B16A1 swap i know i need a new water pump/thermostat but other that that im running alright except when im in nuetral if i rev it up to say 2 grand n let off it drops the rpm's to 300 and then slowly goes back up to 1000 rpm dont know why jus figured ide see what imput you guys had??? any help is appreciated