On startup engine revs at 1500revs after some time itslows to 800revs but is spasmotic

The car "should" rev higher and then idle down. this is normal programmed behavior. However, when the idle is low the car the motor should run smooth. If there is a "miss" like the motor is lurching, replace the plug wires. If there is a rough idle (revs up and down) then look for a vacuum leak starting with hoses between the motor and PCV valve. Anything squishy or oily in the rubber hoses indicates a hole. Also check the air ducts between the air filter box and the throttle body. If you can't find a leak, spray carburetor cleaner around the intake. If the motor revs faster at some point you have found a leak. Also check for a faulty fuel regulator by removing the rubber hose to the regulator to make sure it's dry and contains no fuel in the rubber hose (any fuel means the regulator has failed and requires replacement or repair by someone who can replace the diaphram).