I have three Toto model 703 toilets in my home. in the one on our main floor , I recently replaced the fill valve and float mechanism with an aftermarket unit from the local plumbing supply store ( not a Home Depot or Lowes). While the toilet shuts off now when the water gets to the correct level, I find an almost continual leakage of water into the toilet bowl itself. Every so often the toiler will auto flush.

I've also replaced the flapper and added a seat for the new flapper which came with the kit.

Any ideas as to what might be causing this to happen?

Thanks.