th400 Q

[Kager67]

I have a question about the th400 in my 67. Last fall before I put it away for the winter I took it out for one last drive. Everything was going good until I took off from a stop sign and it wouldn't shift from 2nd to 3rd. I stepped on it and when the rpm's got to around 4 grand it shifted(and shifted hard). Fast forwarding to this spring, it started doing it again. The only thing is now that it doesn't even want to shift out of first without revving it up. My first thought was the modulator, but I hate to just throw parts at it. The other thing that could be a possibility is that it has a aftermarket dipstick in it. It does leak a bit so I have to add a couple times a year. I'm just wondering if it could be off and I am over/under filling it. Just seeing if anyone out there has had the same problem and could give me some advice. Thanks