99 Mustang - loses acceleration

Over the past two or three weeks, at intermittent times and without any apparent common ground, my 99 Mustang (v6, 78k mi) will stop accelerating. I hit the gas, sometimes it will rev, sometimes not, or not convincingly. I pull over. When I bring the car to a full stop, the engine sounds like it's seizing, and I'll shut the car off. However, I don't even have to wait two seconds before I start it up again, and everything's fine until the next time it happens again - two days later, five minutes later - like I said, no common ground such as running hot, idling, taking a turn, etc. I replaced the cooling fan last weekend after the a/c wasn't coming on (or would come on/turn off/come on/turn off). After replacing the fan, the a/c works fine until the problem above occurs, and it too acts up along with the engine - these problems pretty much started at the same time, and I thought both would be fixed by replacing the fan. Thanks in advance for your help.