Well, from the sounds of it, something is really going wrong in that motor of yours. Valves do need occasional adjustments, but only of the slight variety if everything is A-OK inside the motor. If you find yourself needing to make large adjustments, something is probably amiss. And there is no reason you should ever need longer adjusters. If you do, you really need new cam&lifters.

If a valve is continually tightening up, the valve seat in the head is probably receding. This almost always (alway?) happens on the exhaust valves. It's a sort of slow motion disaster, but once the hardened layer of metal wears out of the valve seat (I think they develop a hardened layer when used way back when with leaded gas, but after decades of unleaded...) the valve just starts to slowly eat through the head. There's a lot of metal in a Volvo exhaust port, it could probably keep on eating thru the head for a long while. But as it eats, it tightens up. And a tight vavle both doesn't cool itself against the head as much and, if tight enough, can let hot combustion gasses out, which will result in a burnt valve. So the receding valve isn't an immediate concern, but if you don't stay in front of the ever tightening clearance you'll burn a valve and have to fix is right then and there.

If a valve is continually loosening, that could be signs of a more sinister problem, a sort of Achille's tendon on a pushrod Volvo - the lifters and the cam lobes, and their tempestous relationship. There's a lot of stress and friction on the tips of the cam lobes as the push the lifters up and down, and the only lube they get is a little trickledown from the lifters themselves and the cloud of oil thrashed up by the spinning crankshaft. Some people seem to be able to go hundreeds of thousands of miles without a hiccup, others have recurring rashes of flat lobes and consumed lifters. I think it has something to do with the way the car is driven. Lots of high rpms place greater stresses on the valvetrain (and cause the valve to start receding sooner too). Anyhow, if a valve is continually getting significantly looser, it probably means something is happening to the lifter. The 'classic' failure in this part of the engine is a cam lobe going flat, and the resulting roughened remaining lump of lobe chews away the end of the lifter. By the time the lobe has worn enough to be noticeable while running, it will be very visible to the naked eye. Pull off the valve cover, pull the coil wire, and eyeball the valves as you crank the motor over. If you have a flat lobe, you'll be able to spot it. Assuming it is flat enough to make it run bad, that is.

And finally (should this have been first) this might all just be the result of how you adjusted the valves (although no the part about needing 'longer adjusters' - that sets off all sorts of warning bells). I always use the following procedure, because it ensures you are setting the valve clearance on a valve when the lobe is on the opposite side of the cam. Setting the clearance with the lifter even slightly up a cam lobe will result in it being WAY TOO LOOSE. Basically, on a Volvo motor you can set a valve when the 'mirror image' valve is down. By mirror image, I mean front to back:
#1 - #8
#2 - #7
#3 - #6
#4 - #5
#5 - #4
#6 - #3
#7 - #2
#8 - #1

Supposedly it doesn't matter if this is done cold, warm, or hot. I tend to do it when the engine is cooling down. Because I don't like fighting to turn the motor around as I adjust, scraping my knuckles on the radiator as I tug on the fan, I just pull the coil wire and bump the motor around with the starter. to keep it simple, I just go 1-2-3-4 on the valves. theoretically you could do it with less turns, but it doesn't really matter when you are using the starter anyhow. Just bump it around until #1 goes down, then set #8. Then when #2 goes down, set #7. And so on. I use .018 on mine. Also use a .020, make sure the .018 goes in without resistance, but the .020 fights a little. And be aware that it will tighten up a little when you resnug the lock nut, you might have to fudge a little to get the proper clearance with the nut tightend down.
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I'm JohnMc, and I approved this message.