2000 Sunfire - engine light on/flashing

Ever since it got extremely cold here, the engine light on my car has been on and the car is making a "chugging" noise, like as if it's about to stall but never does (an idle problem). Then while I'm driving my car, the engine light starts to flash when I bring it up to about 50km/hour (35miles/hours). I took my car to the Pontiac dealership where they ran the diagnostic and charged me $400.00 to replace a cracked spark plug cable and the fuel filter. This has not resolved anything! The car is still running like a piece of crap, the engine light is still on, and I know I'm going to get stuck in the middle of nowhere when the car finally dies. Could it be a thermostat problem? The engine refuses to warm up while it's idling, and only does so when you rev the engine to 3 or 4 thousand rpm. Any suggestions? I'm running low on cash.

It is possible that the thermostat is stuck open since it won't warm up. This could be sending an engine temp code to the computer and that is why the light is on. Take the car to Autozone if you have one in your area and have them scan it. They will do it for free. Once you get the codes they will tell you what you need to do to repair it or repost and someone here will help you.