Turbo-400 Trans Slips When Cold

70 Chevelle SS. When the trans is cold/cool, it doesn't want to shift into Reverse for several seconds, sometimes almost 30 sec. Then in Drive, slips/doesn't want to go very well. If you manually shift it into 1st/Low, it immediately starts whining, but will easily drive away. Then you manually upshift to 2nd, and same as in Drive/3rd, the RPM's come up, but not much forward push. (This is all with light throttle settings). If you drive it a few blocks in Low, then it suddenly begins working just fine.
Fluid & filter has been changed regularly/often, I even dropped the valve body, checked all valves, removed accumulator cover, reverse and (I believe) 2nd servo piston. All looks fine, ring is not worn bad at all, springs & balls all there, no varnish. New filter & fluid, even flushed converter.
It was fine, and just kind of started doing this about a month ago. It does get driven fairly regularly, just not a lot of miles. Hate to pull and disassemble/reassemble the entire trans, even more if it still has the same problem. Like I mentioned, it is perfect when warm. Tried adjusting the fluid level up & down a pint at a time: +1, +2, then -1, -2. No change. THe whine is fairly loud in low, but when it warms up and starts acting normal, that sound goes away. Possible front pump? Or an other ideas?
Thanks again! -Dan.

Youy have leaking internal seals, only fix is to do an overhaul.

Second vote for internal seals. It will only do this for so long before it no longer moves.

There may be a short term solution, but only short term. Transmedic has an additive that softens the seals. Lucas also has an additive that is more readily available. As I said, this may be a short term solution. An overhaul is inevitable.