I removed and renewed the hot and cold taps in my bathroom sink. Now the hot water tap shuts itself off. I replaced the brass seat, the 1/4" rubber washer and the o-ring on the shaft. The tap was "firm" when turned on. Any ideas what could be causing this? Any help would be appreciated, my wife is going to kill me.

I had the same problem on a new faucet I installed. Give it some time. The "O" ring will swell a bit and stop the slippage. Good luck, Tom

I agree with Tom's advice but I have a laundry tub fixture that is still shutting itself off after 6 months. I am replacing this soon. Since this happened to an older existing faucet I think you should consider what I am doing.

Yep, I agree with you guys. It is combination of high pressure and new stems. I would try to lower pressure a bit, maybe closing angle stop half way... See if that helps.