`02 4.3 blazer will not start and runs backwards

Hopefully someone can help out here, I have an `02 Blazer what will not start. Good spark, good fuel pressure, injector pulse, cranks fine but hits hard about every 4-8 revolutions and nearly stops for about a half second. About one in every five attempts to start when I let go of the key the motor will try to run backwards. Did some research here and on Google and everything pointed to the crank sensor, so I replaced it with the same thing. No codes are set at all, cam sensor? Thanks in advance for any responses.

Hopefully someone can help out here, I have an `02 Blazer what will not start. Good spark, good fuel pressure, injector pulse, cranks fine but hits hard about every 4-8 revolutions and nearly stops for about a half second. About one in every five attempts to start when I let go of the key the motor will try to run backwards. Did some research here and on Google and everything pointed to the crank sensor, so I replaced it with the same thing. No codes are set at all, cam sensor? Thanks in advance for any responses.Did you do any work to this engine? Was it running and then just quit? Please provide as much information as you can so we might help you.

Will it start with a shot of carb cleaner into the intake?

No work was done, just one morning it would not start, and it will not start with starting fluid.

Maybe a blown head gasket? Getting coolant in one or more cylinders causing excessive compression.