car heater has mood swings 1990 mazda mpv?

[derekastregal]

ok i have an old mazda mpv. still runs decent even though it burn alot of oil and has a small leak in the gas tank which im looking for a new tank as i type this. so anyway im in chgo so we have really cold winters here. my ac does not work anymore but when i turn on the heater it sometimes feels like the air flow is not as powerful as it use t be and also it will blow cold air and then as i drive it will blow warmer air but sometimes go back to just luke warm or cold air. so is this just cause its an old car or is there something i can do to fix this. can a car low on oil make t blow colder and wnen there is more oil in it will be warmer?
new spark plugs?
i dont know all i know is its gnna be 10 dgrees tomm and im gnna freeze my but off