I have an issue with trans to flex plate

Hi guys I have an external balance 360 with the b&m flex plate that lets me run an a904, which has been working well for a number of years, I also have the 148 tooth ring gear on the converter so use the odd starter.
My issue is that recently I started having starter engagement issues in that occasionally it would grind and not engage with the ring gear teeth, after burning out the factory style reduction drive starter I had I bought a mini starter for it which initially worked for a few months, I have checked engagement and it seems fine.
What I have found is that winding the motor over by hand it gets to a point where it goes tight and really needs some pressure to continue to be rotated, even removing the spark plugs doesn’t make any difference, I disconnected the torque converter from the flex plate and the motor turns over easily by hand, and so does the transmission until I put a bolt into it again to re engage the converter completely.
Any ideas where in the transmission I would be looking, I have not pulled it yet, but it was completely rebuilt back in 2012 and would be lucky to have had 5000 miles put on both the motor and trans since being built, by the way both are fairly stock builds the motor is 40 thou over size and stock heads with a small cam that performs mainly in the 2500 to 3000 rpm range and a converter stall of 2000 rpm and had the transgo stage 2 modification done to it.
Thanks for reading the novel