small block chevy help

i have a 71 nova, 350, i keep having a problem with it and cant figure it out without some fresh ideas. My car was running 14s, inital advance was at 21 but thats what it needs, i got it to run a 12. 80 and it pulled real hard by advancing it just from like 19 to 21 initial. i then brought it home after running good times and drove it about a week later, still pulled great, drove it today and its a 14 second car again. I have absolutely not touched anything on it since it ran great, what can be goin on, could it be something in the distributor screwing everything up? possibly carb? what could make it run worse by just starting it up a day later and not touching anything, it sits inside and valves were just adjusted, it will act like this a lot, run slow then just run great for a day or two. It doesnt sit outside so i doubt its water, if anybody gots any ideas let me know if possible, when i shut my car off or start it up could my distributor be moving or anything, let me know if anyone needs more info on the motor. thanks

im just basing my "14 second" pass on feel of the car, i havent ran it since then so i ruled out all of the race stuff. im not spinning and the reason i know it runs good is based on pull and i usually blow my v belt at about 90. what does aluminum intake have to do with anything? i got a team g. I really think its in the distributor, i pulled it apart and now i noticed the little metal tab off the rotor waas rubbing on the side of the little thing that comes out of the cap under the coil. it actually wore away the plastic of the cap, how key is that to performance if its doing that? thanks again.

thanks for the help, ill try to get it out when it dont rain around here and see what happens and if anything i did helped which i doubt it did, plus im gonna try to find that "DUHmie" that put that cap on, thanks again.

should update this

found out what it was, weights in distributor were sticking, sandpaper and a real good cleaning took care of that. thanks for the input