The cars cranks but does not start

Well my car just stopped working basically stalled as I spammed the brakes repeatedly and now it just cranks and even if I manage to turn it on the rpm just jumps up and down the 1 range and turns off like it ran out of fuel but the injectors work everything is working took it to a workshop they said its the security module but the immobiliser light turns off when key is inserted as I bought a key smith to duplicate a key and program it and the key smith said it was not the security system it's something else these mechanics are trying to scam what could it be?