^ I think you need to chill out with the bashing, because you seem to taking it way to serious.
And Russell is right, It probably doesnt have anything to do with the coolant system. You could own 100,000 MR2's and still know nothing about how cars work. There is nothing in a cooling system that would prevent a car from starting. Even if the sensors were all bad, it will still start. Its true that the ECU would go into "default mode" if one of the sensors were bad, but that would just keep the Cold start injector from firing...... which means it wouldnt start in the cold.
Check your Throttle position sensor. Variation in temperature can cause the resistence to change in a TPS, especially if its malfuctioning or has moisture buildup. That may be one of the reasons for having a hard time starting it.
The shuddering and stalling could be a Vacuum leak. These engines are very prone to that. Especially when warmed up. An air Bubble in the coolant system could cause an irratic or bouncing idle. but stalling....no