1990 Sedan Deville -- Headaches !

I'm driving a 1990 Sedan Deville with 102,255 miles and a list of headaches. First off, I replaced the alternator, radiator, thermostat, egr valve, egr solenoid, harmonic balancer, water pump, fuel injectors, and recently gave this girl a tune-up. When I'm driving the car (stepping on the gas) this car runs with the best of em, but when I slow and stop for example, at a stop light, this car shakes and feels like its going to "die" on me. When it's idling the car shakes and runs rough. Thats if the car even "feels" like starting that day. Recently, when I go out to start the engine the car does nothing. The lights on the dash come on, the lights all work fine, the battery (yellow optima) has been checked, yet the car doesnt turn on. Then after cursing loudly at this "girl" and calming down, trying to start the car it turns on like nothing ever happened. I can't even drive this car because its unpredictable when/if the car will even start. I have an alarm with an automatic start, sometimes the alarm doesnt work properly and I'm thinking maybe the automatic start connected to the starter may be whats causing this problem. Also, going back to around 90,000 miles, the car started after turning the key 2-3 times. So I need help with this car . What could be causing these problems I have? Is this car even worth waisting my time and money on? If everything was based on looks then this car would be worth every penny. But if the problems ahead for this car are more than the car itself then off to the automobile cemetary for me :crybaby: