A320 pilot reported a fuel anomaly during cruise which returned to normal; flight landed uneventfully.

Narrative

On Flight ABCD; we experienced a fuel anomaly with no ECAM or any abnormal indications where the fuel fluctuated about 3000 pounds. It dropped from where we expected it to be down and then cycled back and forth for probably about 40 minutes and caused the FOB between MCDUs (Multipurpose Control Display Unit) to read differently. The FO started keeping a fuel log based on fuel burned as it seemed to be the most accurate. After this amount of time; the fuel cycling stopped and all readings returned to normal; including the MCDUs; and the landing in ZZZ was uneventful. We elected to write the anomaly up and have MX (Maintenance) take a look once on the ground. MX was called as we were taxing in to meet the aircraft and boarding for the flight back to ZZZ1 commenced on schedule. After witnessing exactly what we had seen in the air (we had made a video to show MX but the problem also presented while we were on the ground in front of MX); MX tested the fuel system and found the error was presenting in a fuel computer. MX said they had only one part on station and it was brought to the aircraft and replaced swiftly. Unfortunately; once the part was in place it was not able to be tested; and could not be verified as working properly. MX chose to replace the new computer with the old bad computer and mentioned to the pilots that if it tested; it was good to take. The old computer was put back into place and it did test; however we did not feel comfortable taking the part that they determined was faulty in the first place. After determining we weren't going to take the aircraft; it was taken out of service and company was notified that we will be deplaning and searching for an alternative solution for the flight. Soon after; it was determined that we would take an aircraft that was arriving from ZZZ2 and continue the flight to ZZZ1. Once the aircraft arrived; the passengers were boarded and the flight continued to ZZZ1 uneventfully. Passengers and crew were kept up to date throughout the situation by a combination of CA and the company.