What color is the smoke from the exhaust pipe? Black smoke is fuel from being flooded. White smoke is water and light grey smoke can be oil. If it is burning coolant, besides the white smoke at startup, the smoke out the exhaust will have a "sweet" smell to it besides being white smoke. I can not describe the smell other then "sweet" but it is a different smell and is caused by the anti-freeze burning that is mixed in your coolant. The longer you leave the off, the worse the start problem? If you restart the car in a short amount of time does it start alright? If you leave the car sit for a day or two the start up is worse? This can be coolant leaking into a cylinder over time but could also be caused by leaking fuel injector/s, once again the color of the smoke can help.
Temperature gauge jumping around can be caused by an air pocket in the cooling system. This must be fixed before more damage is done to the engine. You have to find out what pushed the air into the coolant system. It is easy to get an air pocket in the cooling system of these cars if you have had the coolant drained from it. If you have not had work done to the car's that required the coolant to be drained down then check your engines cylinder compression on each cylinder and compare the compression readings. Poor compression on one or two cylinders can be one indication of a bad head gasket. Further testing by looking for exhaust gases in the cooling system can also lead to a bad gasket or cracked cylinder head if found. Any coolant leaks? Have you had to add coolant? Is your pressure cap working properly? When was the last time the coolant was changed? Has the engine been overheated? Pressure test the cooling system and the fuel system can also be clues as to which system is having problems.
You need to start to trouble shooting the problem and stop guessing and replacing parts. It is all the little clues that you need to start homing it on what the problem is. If you do not have the tools or equipment for testing then you are stuck with guessing, which can be very expensive. All anyone here can do is guess by what information you provide.