Overheating

The car started pushing coolant out of the overflow tank and did not suck it back in and overheat. We pressure tested to cooling system and it held pressure. There is no sign of coolant in oil or oil in coolant. The car is not skipping and it runs prefect. There are not any leaks on the motor. The motor lost spark above 4800 rpms but we tried a msd box on it and it covered it up but it did not fix it. I bought a msd distributor to go in after i fix the cooling issue. please help with both issues. mods are h/c/i with vortech and upr power pipe. I also put in new plugs wires and coil each one pick up power but the msd gave me the rpm's back.:shrug:
 
change the thermostat. i had the same problem. my car was running hot and when i pulled over the overflow was full and pushing out of the little hole towards the top. i started a thread and was told the thermostat was either not opening or not opening all the way.