My 94 Buick LeSabre misfired almost constantly on cylinder 6 and also
misfired a lot on 4, but seldom on others. I got error 300.

I replaced the spark plugs and wires and it did the same thing.

I have the Actron ScanTool that tells me what the car is doing and during
the same space in time 1,2,3,5 were firing  but 4 and 6 didn't.

The misfiring happened randomly.

I also replaced the Ignition Module and Coils with the ones from my
Bonneville that works fine. I switched the injector 4 and 6 with the
injectors 1 and 3. Fuel pressure is normal. The same cylinders as before
keep misfiring. 4 and 6 were the worst and they still are. Any idea what may
be causing this?

From bad to worst.

Then, this morning, the engine was running as yesterday, misfiring but
running. I started the engine to check it again with my ScanTool. A few
minutes later it stalled and didn't start again. I have the original
Computer that went bad maybe 2 or 3 years ago. The engine worked rough with
it but worked; I tried it before yesterday and the engine started and run,
rough but run.

So, guessing that maybe the Computer went bad (I had bought it used) I
replaced it with the original I had from before, but the car didn't start
either. So I am assuming that the non-start thing its not the Computer, but
I have no idea what to look for next.

I checked, again, the fuel pressure, for spark in the spark plugs, for
disconnected vacuum hoses. Fuel pressure is about 42 psi, there is spark,
and I didn't see signs of vacuum leaks.

Trying to fix this non-start condition I replaced the Crankshaft Sensor and
the Camshaft Sensor. Nothing!

When it stalled there were no noises or anything else, it just stopped
running.

Could it have anything to do with the security system?