Ok I think it was Jim tn who said he thinks it could be the coil (11"), got so many suggestion can't remember them all, but the only other coil I have for the F75 is the 5" that came with it. Disc 25 Foil, 85 SEN, 3H tone( Hi-tone for nickels) BP process. UNBELIEVABLE!!! it was more stable then my 1270, and nickels came in at 8"s. it was stable smooth not erratic at all, ID's were dead on the money, even in the area I call CAN-SLAW ALLEY, It worked perfect, but I had that same thing happen with my 1270 went crazy for 2 days called Fisher for repair didn't sent in in right away and the problem just stopped for 3 weeks then it came back they replaced the ground balance pot and recalibrated it, that last 2 months just sent it back for the same thing. Posted a pic of todays finds with the 5" coil. Now putting the 11' back on it and trying it out.