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The problem is bad slippage in Normal drive. So bad that the car wont run above 5mph or so.
This happens all the time, when trans is cold or warm, hitting the gas pedal right off the start from idling. Other selections such as L1 and L2, R work fine. Only the D slips.

I have an original TH400 from a 1970 Cadillac Fleetwood with a 472cu engine.

I did drop the pan, changed the filter & fluid, and checked the transmission linkage.
After driving for 10 miles on Low, I dropped and checked the pan again and found a few black rubber chips in the pan... not sure if this is related.
I did check the fluid level many times, warm and idling on P, fluid level is fine and not leaking outside.
Also, when engaging into normal drive, the transmission makes a "klung" sound and kind of shakes the car harder than other selections.

Any thoughts ?
Thank you.
GM  

The forward clutch is probably shot.
The reason L1 & L2 work is because there is slightly higher pressure in those ranges.
Reverse uses different clutches.

Time to break out the wrenches.  

Stuart, thanks a lot for your answer...
GM