Hi,
Slipping and grinding is the sign of a worn out transmission. If it was not maintained properly then most likely the seals internal to the unit have failed and are letting pressure by which causes the slipping. The grinding is due to pressure going to places it should not and causes gears to clash. You holding the accelerator up to put "pressure'" on the transmission is you keeping the pressure higher which is overcoming the cut/leaking seals.
Unfortunately, if it were me I would be tearing it apart and rebuilding it. More than likely you will find metal in the pan which would be another reason for a rebuild or replacement.
Let me know if you have questions. Thanks
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Thursday, September 19th, 2019 AT 5:07 PM