Last week 3/4 of the way thru a 10 hour trip my beloved prize became haunted by demons. Stopped for fuel and found while getting back on the road that the xmission decided to not shift {and very harsh when it does} till about 3200 rpm 1-2-3 and wont go into 4th at all unless I go to neutral and then back to drive, then it goes into 3rd for a second and then 4th. This is a xmission that has never had any issues and had a fluid and filter change about 20,000 miles ago. I have searched the posts for 2 days and checked everything I could find. Fluid level right on the top mark hot. Unhooked the kickdown at the pedal and also the xmission, checked the vac- 12 in at the feed to the modulator which drops to near 0 as throttle is opened, modulator holds 12 inches on the mighty-vac for as long as i cared to watch it. No other vac leaks found {envy me??} I think that I saw in my search that a blocked filter causes some kind of pressure rise but I can't find it again. Does anyone think a fluid and filter change might help or would I just be barking up the wrong tree?

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83 300SD- 223,000 miles {mine}
85 300D- 187,000, {wifes}
82 300D- 374,000, {parts car now, my first diesel}
74 Triumph Spitfire {constant PIA}
72 Jeep Commando {snow ride}