"Without the collet and using the large bits we're all good, no slippage."
By that statement I think you are using a reducer bushing for the 1/4" shanks? First question is has it always slipped or did it just start? If it has always slipped then toss it and get a different one. If it just started slipping then it needs cleaning according to Stick's link or it is worn out. Check the mating surfaces of bushing to bit and make sure it isn't galled from wear and heat from the slippage. If it is you definitely need a new one. I think there is a Musclechuck to fit that router and it should provide a better grip than your regular collet.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.