Hello and welcome to Just Answer!
When you say it slips, does the engine rpm flare up like you put the shifter into neutral or does it drop down to a lower gear?
Is there any whining noise when you shift into gear?
It sounds like you have low line pressure, this could be caused by the sump filter plugging again, an internal sealing issue or a sticking pressure regulator valve. A very common problem I see with these 46re transmissions is the converter clutch will burn out and when this happens, it will start to plug the transmission oil cooler and fill the filter full of clutch material.
I hate to tell you this but, the transmission will need to come out for repair. Anytime the transmission is slipping, it doesn't take long to burn a clutch pack or band out. I wish I had better news for you, but if the transmission is slipping, there will be damage to the clutches and bands.
I hope this helps, let me know!
The average cost to rebuild this transmission is around $2500.00 and that includes all parts and labor.
Do you have anymore questions, let me know!