I have read now already several threads about this on US forums and also on German forums, where also the E36 has a 5HP18, here copied from info, so this basically refers to all 5HP, apparently later they changed the material:

I have a 525i 1993, start notice that after driving for 30 min the shifter became hard when I went to park or drive, I decide to change the trans oil in a service shop. Same day I took it from the garage I tried to park at home and again the shifter didnt let me to shift to P. Then I push it and suddenly the shifter went back and forward easily but none gear works. The mechanic went home and told me that the shifter cable was the problem, change it but still the same, no gears working... Again back to the service shop, they found that the manual valve of the body valve was broken (ZF part 1056-327-173), change it for a second hand part and still with the problem. Now the mechanic said that must find a new manual valve to replace it. Notice when the trans is cold I can use the shifter without problems and the car and gears drives well...until it became hot. I already bought the ZF manual valve but before install it I need an advice because I feel that something is wrong with the temperature of the transmission.
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The manual valve on the 5HP series is known stick and then break. It only costs about £12 so I am not sure why anyone would fit a used one. Fit the new one and make sure the end is hooked up correctly. If you don't, you will get a box full of nothing
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Thanks Wiskeychaser, the mechanic just found this piece and installed without notice ...well here is imposible to get one of these parts but finally got it on internet, hope this will be the reason why the shifter get stick..let you know.
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When I got my 1994 530it earlier this year, the shifter was definitely difficult to move. It finally just snapped the head off. I tried to fix it, with an obvious failure. The dirty ATF had made it brittle. $16 later, I had a brand new one in the mail. Haven't had an issue since. If the car sits for a long time or has the same fluid in certain climates, the end of the plunger swells. I actually sanded the original one down and polished it but it still managed to break. The new one works flawlessly.
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I just read about a similar case on a German Forum, E36 M3 3.0 with 5HP18. Completely broken, apparently in older 5HP18 and 19 this part was made out of a kind of 'plastic'. It started with heavy shifting and then the part cpl. broke into 2 pieces, one part was falling out after he removed the trans pan, rest sticks in valve body.
Selector Valve http://www.thectsc.com/products/sele...alve-84-3.html 5HP18-5HP18A-5HP19-5HP19FL-5HP19FLA-5HP19HLA-5HP24-5HP24A-5HP30 Price: $18.50 Product ID : 1056 327 173 01

ZF5HP18, Valve, Manual (1993-Up) (1056-327-173) https://cobratransmission.com/index....1497_1558_2387

this part is also included in a new 5HP30 Valve Body Kit - 5HP30VBK http://www.zftranspart.com/index.php...t_version=true