3.9 efi efi ecu problem

[Big woof]

Hi there all I got another ecu for my car out off 1997 discovery which is still the 14 cux 3.9 v8 but just socketed for a chip. Only I think that either some one has had a go at re chipping the car with a super chip. Which I am thinking has now gone faulty I have done just about every sensor on the car which is controlled by the efi and yet it has moments off brilliance and then it all off a sudden it will rev to 6 thousand Rpm the next day or four hours later it will only rev to 5200 rpm.
Now I am only thinking that its the fuel chip but I could be wrong I have had my snap on scanner plugged in and everything reads normal a mate had rover gauge plugged in and it's still saying no fault codes and all sensors are fine the ignition system has been checked and double checked
On the advancement off the engine and all is normal. Even down to the contact in the distributor is with in tolerance. My next question is where can I buy just the chip for the ecu either std form or performance im not bothered which one i just want it running right and lovely.
Thanks for micheal