I have a 20 foot flagpole antenna fed with about 100 feet of RG8 coax. There are plenty of ground radials as well as grounding rods. Grounds are all bonded with house electrical system. I was using an LDG 600 RT remote tuner and it worked very well to tune the antenna, however, I had a number of heat related failures so I decided to try out the MFJ-994BRT tuner. The tuner actually tunes the antenna quite nicely, however, at higher powers (over 75 watts) I am getting a high SWR spike momentarily when keying the transmitter. It is intermittent, but causes my ALS600S amp to fault. Even without the amp, I've noticed that the radio folds back power momentarily when these spikes occur. I've tried several things including using a current balun between the tuner and the vertical. I've tried using the "random wire terminal" on the tuner. I've tried all HF bands including 10MHZ where the antenna is most resonant. Any ideas or suggestions would be welcome.