Idle Problems

My Nissan 89 Pulsar is driving me crazy. It's the erratic idle problem. It was supposed to idle at 800 RPM and for past 1month it has been idling between 1300RPM and 1500RPM. Whenever I switch on my headlights and heater it sometimes (but not everytime) drops to 1200RPM. I changed my Oxygen Sensor, Spark Plugs checked for vacuum leak found a crack in one the tubes and changed it but still the problem is not gone. When I changed O2 sensor I have seen carbon deposit in exhaust manifold (Which is expected as the engine is running rich) I haven't checked for any seal leaks yet. And since the problem started I am also having 5th gear (by the way my car in a manual) popout. I changed the transmission oil and found no metal in it and I am guessing this is due to the engine pulsating sometimes. I haven't checked the stick and other links because I was hoping that I will address it once I fix the idle problem if needed. I am guessing that there is some electric problem because sometimes my headlight high beam turns on for few seconds and gets back to normal. Is this something to do with a relay or some OBD1 codes. I have not scanned the Computer yet? Do I need to? If yes where can I get it done cheaply. Please Help me

Idle Air Control Valve is your problem. Carbon buildup inside it causes the car to idle very high(it is basically like an electronic choke) and Nissan/Honda's are very common for it to have happen.

Remove the IACV and clean it with some throttle body cleaner.

www.pga.org.au should have some more info on how to clean it out for you.