Firebird v6 multiple missfire p300 code and trouble starting.

P300 is a random multiple cylinder misfire. I would start at having your DIS module tested.

https://www.obd-codes.com/p0300

P0300 OBD II Trouble Code Random Multiple Cylinder Misfire Detected...


Autolite Challenge Series P0300 Engine Misfire Diagnosis

code p300 refers to random/multiple misfires--misfire detected==causes--spark plugs--HT leads --injectors--ignition coils--low compression--wiring
If you are only now experiencing the problem after changing the ECU, I would suspect that wiring and connections to the new ECU may not have been fitted properly. There may be problems in the ECU CPU (chip) and it may be necessary to use a scanner to reset the CPU program to recognize the sensors of the vehicle correctly

Change...distributor cap ..spark plug wires or spark plugs

this usually indicates multiple missfires in the motor and not just any particular cylinder.

Please see my tip at http://www.fixya.com/cars/r6715318-causes_gasoline_engine_misfires . These are generic instructions. If you get stuck or need specific instructions for your car, please get back to me with model, year and engine info.

Coil packs are labeled or aren't labeled? Firing order is 1-6-5-4-3-2

Error code P0300 means that you are having multiple/random cylinder misfires.  I would start with changing all of the spark plugs and see if that will fix it.  You will have to have the code reset at an auto parts store.  If that doesn't work let me know and we can progress down the line.

look up the codes if they are multiple missfire codes then more than likly you have a bad head gasket

It could be a clogged fuel filter, bad gas. Or if you have done alot of modifications to it, the injectors may not be able to keep up.