'94 940 turbo kickdown cable/valve body question

I have a 1994 Volvo 940 turbo with a problem. Any advice anyone can give me will be appreciated. This is a high-mileage car (217K) with a transmission that
was fully rebuilt 43,000 miles ago.
The problem started when the accelerator pedal started sticking. It'd go down a few inches, stop, and wouldn't free up unless I stomped on it. Other than that, things seemed fairly normal (acceleration up to the sticking point was normal).
I opened the hood, and the problem seemed to be the kickdown cable was getting hung up on a bolt end. When freed, however, the cable did not properly retract at idle or when the engine was off. It had about an inch or so of slack in it, and the cable refused to retract past that point (even when forced).
I looked in the Chilton's and Bentley manuals (all I have), which told me that replacing the kickdown cable required opening up the auto transmission case. So I took it to the shop. I should say I'm a fairly good backyard mechanic, but have never gotten into an auto trans case (I consider them a species of voodoo, perhaps this is just my own fear, though).
The shop replaced the cable with a new OEM cable. They said it was fixed, I paid, and I drove away. Within 0.25 miles, however, the problem returned -- the sticking happened again. So I turned around and drove it back. For the next 1.5 days, they basically scratched their heads and didn't know what to do. Finally, they determined that the kickdown cam inside the trans was not perfectly circular, and took the cam off and ground it down until it was perfectly circular (this is what they told me). They told me it was going to be loose, and that I'd eventually have to replace the valve body (which would also give me a new kickdown cam).
They gave me the car back, and it is still having problems. The kickdown cable is now slack (by 0.5 to a full inch), when the motor is either off or idling. When pressing down the accelerator pedal, you get an inch or so of free travel, then it sticks again (not on the bolt, as before), which can only be solved by stomping on the pedal. Driving the car is a completely different experience now. Instead of being peppy as heck (love that turbo), it now feels like a 4-cyl. Ford Escort. The pedal sticking is part of this, but the trans itself seems to have been affected somehow.
This is not a problem with the pedal, or the throttle body (and probably not with the new cable). When the car is off or idling, when you manually pull on the kickdown cable, you get an inch or so of smooth travel, but then you hit a sticking point which can only be overcome by increasing the pressure significantly. This seems to indicate that the problem is within the transmission, specifically the cam itself.
So, I have a few questions. Taking it back to the shop isn't really an option, because I have lost a lot of trust with them.
(1) Should I try removing the slack in the kickdown cable? This is something well within my mechanical abilities, so it's the first thing I'm thinking of trying. Will this affect the trans during intial acceleration or when it should kick down? (2) If the cam is the real problem, is the answer getting a new valve body? How much can I expect to pay for a new one? Is getting one from a junkyard a viable alternative? (3) How hard would it be to replace the valve body? It seems to be fairly straightforward, but then I don't know much of anything about automatic transmissions, so that could be incredibly naive, I realize.
From what I can see, the trans oil pan seems to be accessible without fooling with the engine or transmission mounts (meaning the whole job wouldn't involve dismounting either). So, pull the oil pan, remove the filter assembly, unhook the kickdown cable, and it seems to be a question of just unbolting the valve body and replacing it with a new part.
Would I have to unhook and remove the gear selector shaft? It looks like it is independent of this job, but I could be reading that wrong. Is it possible to do this job without completely redoing the kickdown cable replacement (in other words: can I unhook the cable end within the tranny case without having to restring the whole cable)?
What I'm really asking is: am I deluding myself thinking that this is within my abilities?
It seems pretty straightforward. But then, I know nothing of the mysteries within the auto trans case. So, are there hidden pitfalls or extra experience necessary? Are there tiny adjustments which must be made of which I am unaware? Any other reasons why this is not the straightforward job it appears? Please, I would love to know....
Any and all advice would be apprecitated! I would really like this car to get back to its formerly aggressive self, so that when I step on the gas pedal it travels smoothly and gives me lots of oomph... and I REALLY want to get rid of the annoying sticking problem...
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