Topband: Tuner ??? on 160

[K0YR]

I was playing in the 160 last weekend and experienced some problems with my setup when I tried to run power. I have a 160 dipole fed with ladder line and a Vectronics 1500 tuner. I used it all  Friday night at a 100 watts and did not have any problems tuning. On Saturday I put one of my amps that will do 160 in place. When I went over 200 watts, the tuner would start to bark and arc across one of the tuning capacitors. I tuned it up on another 40 meter antenna and my th11 beam and it worked at full power fb. So any thoughts as to why it will tune up nicely on 100 watts, but will bark at anything more???
Tom, K0YR