Tranny slipping?

I have an 02 Foreman 450 ES that I just put some 589 M/T LSW tires and wheels on. I love the tires, but they are just so heavy at 32 lbs. each. So anyway, I was driving it and all of a sudden when I would punch the throttle in any gear, the tranny would slip. I changed the oil and found some metal pieces in it. I try and ride it now and it seems to have no problem except if I am riding on a long straight trail, all of a sudden the gear selecter will start flashing and I can't shift unless I turn it off and on again????? I'm wondering if I screwed up my clutch or if there is some other issue with the quad. Any suggestions would be greatly appreciated.