Throttle kick down cable could be out of adjustment or sticking, that will cause delayed shifting in any gear. I would do a flush of the tranny using synthetic fluid if you have not already done so. A flush cured a non functioning OD on my old 740 after I exhausted all electrical causes including a new solonoid. I doubt the problem is electrical.
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David Hunter

"I have changed the filter/screen* and fluid,..."

If the only fluid changed was what drained from the pan, that was only about 2 quarts of the total 7+ quarts in the system. Given the mileage, your AT (and OD solenoid) might benefit from a complete flush.

The procedure is pretty simple, and is described in the 700/900 FAQ feature. The hardest part will be disconnecting the AT fluid return line at the upper radiator fitting. It will resist your efforts, and you must take care to counter-hold the larger hex head on the (plastic tank) radiator side.

* Volvo no longer recommends this, per the following Service Bulletin excerpt:

=====>
TSB 17106
Dec 1990

"ATF filter cleaning deleted"

for 200/700/900...for all those AW 70/70l/71/72L transmissions and for ZFHP22
(Does NOT apply to BW*)

"It is NO LONGER REQUIRED [capitalized in original] to clean and inspect the Automatic Transmission Filter..." [The tiny filter screen, which you get at, of course, only with SUBSTANTIAL effort.]

Advised to still replace the ATF fluid at specified intervals, and to flush the transmission cooling system.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

I may be a bit out of my depth here so anyone who knows better, feel free to correct me. I had similar symptoms with an OD on an M46 transmission, caused by worn o-rings on the OD pump piston. With the M46 the OD pump is driven with a ring that surrounds an eccentric lobe on the output shaft, easy to get at when the OD is removed. I have no idea, though, if the AW70/71s operate the same way. Fortunately I haven't had to tear one apart yet. I replaced those 2 o-rings for $0.09 apiece from the local hardware store, reassembled, and drove it another 40,000km before the input shaft bearing died.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf

As you mention, this problem comes up a LOT. Used to be "high NOx" was the leader, or so it seemed, but lately it's non working OD.

I think it's a bit awkward to get at. It's tucked away on driver side and sits on top ... finally after some searching I found the old link I have. Not my pics but an excellent shot showing the OD solenoid:

Pic OD solenoid
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Norm Cook Vancouver BC; 1989 745T 204,000KM