When my engine is cold and i try to start it.. it starts fine... and then i use my car and get the engine in a normal temperature and lookin like this on our thermostat

C\ \ | /H

Then when i turn my car off... and i come back to it in 20-30 minutes.. my car has a hard time starting... It cranks.. I changed mah battery, ignition coil, ignitor, timing belt, rotors and cap, spark plug, wires, fuel filter, fuel pump relay.. I was wondering if anyone had this problem with der teg bfore..??i've spent too much just to get fix this problem.. anyone help me plz!!