first off i have a d16z6 in my 89 civic hatch. and my radiator slowly pressurises and all the coolant goes back into the overflow after like an hour of driving, my car doesnt overheat until its low on coolant and also my cars idle drops too 900 rpms and shoots back up to 1500 rpms and it does this all the time, like im reving it, and also when i try to adjust my idle screw it does nothing at all. ive already bled my coolant system, im not throwing any cels, theres no leaks in my coolant system either. ive also cleaned my IACV. any other ideas? im confused, im guessing maybe a head gasket?