HELP!

My 1999 Mercury Cougar is having trouble starting up, usually on cold days. The engine will, once started, rev on it's own instead of idling. The engine misfires and causes slight trembling felt inside the car/steering wheel. The speedometer has always been an issue as well - when first starting the car, the speedometer will jump to 180mph then come back down to 0, then idle at 1 or 1.5. The car recently had a diagnostic work up at AAMCO. The mechanic stated that the intake manifold needed to be replaced, once replaced another diagnostic workup should follow because replacing the intake manifold may not solve our problems. Is it worth repairing? The mechanic flat out said to sell the car and "take the money and run." What should we do!