Drop in RPM

[six is enough]

Car is an 87 GN TA motor with 400 transmission. When car is shifted Rpms drop 1600 on all shifts. This started this last racing season, before they would drop 600 to 700. Tried three different converters and all did the same thing someone thought the problem might be in the transmission made sense as the sixty foots went from 1.30s to 1.50s. Car seems to pull good thur the quarter mile but leaving some on the table as it takes time to pull back up in the power band. Any Ideas?