Won't start below 30 degrees

If you are getting no spark at that temp, check the coil pack. If you are getting spark look to see if the fuel filter is clogged or maybe a failing fuel pump you will need to test the pressure when it hits 30 degrees.

may be temp sensor for computer not the gauge, scan engine in am b4 starting when engine is at ambient temp. all temp sensors should read identically. this is one possibility . if engine sensor reads wrong its as if the the choke on a carburetor is open. This may not set a code. The fact you mention a certian temp. when it happens directs me to check this. unplug sensor and visually look at connector for coolant presence. there has been instances coolant leaked from sensor thru wire into ECM.. rare. leaking sensor happens and will effect signal.