Dart swinger slowing down???

Hello, I'm trying to figure out what's wrong with my dart swinger it has a mild 408 with 727 manual valvebody 4,000 stall 8.75 391. The car ran its best of 11.97 @ 113.95. I ran this pass 3 weeks ago and came back the following and ran 11.97 again, then 2nd pass 12.1 then 3rd 12.50 @ 107 Since then I knew something is jacked up. I checked spark, compression test and timing, all are good. No visible damage to valve train, car starts Every time. I just went back 1 day ago and first pass it ran 12.45 @ 107 Then 2nd pass 12.80! Wtf The car feels like it just not accelerating, loseing hp or something slipping? Please help me, thanks everyone!