My 83's M46 overdrive didn't work for a long time. I filled and emptied the fluid and cleaned the screens. It still didn't work. Then a couple months later it worked but was rough when going into it. It didn't really work all the time and was like yours where if I let up on the gas it would go into OD but then slip out at times.

This past week I replaced the cheap scan tech clutch cable with a GEMO brand one and topped off my fluid which was a hair low since I ran out of fluid last time filling it up. I didn't really put too much fluid in but the transmission and clutch pedal are like new. The transmission is so smooth now and it switches into OD really easy and smooth too now.

I'd clean the two screens. One is the rectangular one and the other a round tubular one under the round cover on the drivers side. I think there are three of these round covers with two little indents for the right type wrench. You need a special type tool to get the tubular one out. I got a tool at Harbor Freight for like $6. But if you have a clutch cable without a metal sleeve to it then I'd switch it out and make sure the transmission is topped off to overflowing. Good Luck.