1998 Toyota Corolla Engine will not rev

Twice in the last month while driving along my Corolla has decided that it doesn't want to rev. It will idle, but the engine won't rev. The first time I was leaving a stop light I had just gotten the car rolling and it wouldn't accelerate. I tried letting off the accelerator and depressing it more easily but still no acceleration. I then put the car in neutral as it coasted at two miles an hour and held down the throttle but even with no load the engine won't rev. About a minute later when I got to a parking lot and turned in to bet of the road the engine started to respond and rev, and I was able to drive home. A couple weeks later a similar problem, I was trying to accelerate from 25 to 30 miles per hour and the car just kept moving at the same speed. I throw the car in neutral and tried to rev the engine but it wouldn't, then about 30 seconds later the engine started to rev again and I was able to keep moving along. The engine did not stall either time. Is there any problem common to the corolla that could cause this?