Power Seatbelts work and sometimes don't work

[etjon]

I have a 91 Mazda lx 626. There are times when I open the door (either driver's side or passanger side) the seatbelts won't work. But there are other times that they will. All other electronic's work w/o any problem. I have power windows, power sunroof. Just don't know what the problem is. I usually get them to start working again by having them unbuckled while driving and they'll work when the seat belt light and the buzzer goes off. Sometimes this works but sometimes not. Does anybody have any answers or possibilities. The fuse box is fine under the dash and under the hood. Help please!