An EGM recording system in an implantable medical device of the type comprising an implantable pulse generator (IPG) and a lead system including one or more sensing leads each having a proximal end coupled to the IPG and a distal end with at least one pace/sense electrode in contact with a patient's heart. A short interval counter (SIC) is incremented by oversense events occurring within a short interval (SI), that is shorter than typical fibrillation event intervals, following a preceding paced or sensed event (except for certain ventricular event sequences). At the same time, the near-field and/or far-field EGM is detected, digitized and temporarily stored in a FIFO buffer as EGM data. When the first oversense event is detected within the SI, a time limited epoch of the corresponding EGM data is stored in a register in a rolling buffer. Subsequent EGM epochs with date/time stamps are stored in other registers of the rolling buffer at programmed counts of the SIC on a FIFO basis. Associated data, e.g., a date/time stamp of the recorded event and the oversense interval triggering storage, may also be recorded in similar buffers. The stored EGM epochs and associated data are later telemetered out to an external programmer for analysis. Lead integrity problems (including loose connections of the lead to the IPG) or electromagnetic interference (EMI) causing oversensing by the sense amplifiers can be diagnosed from the EGM epochs and associated stored data.