I would start with the simple and cheapest.

'99 and '00 FWD/AWD volvos are prone to the electronic throttle module (ETM) failure. So If you are mechanically inclined,

Remove the throttle body/ETM from the car and give it a good cleaning with carb cleaner, avoid spraying the electrical components. Some owners have delayed the dreaded ETM failure by religiously cleaning the throttle body every 6 mos or so. This has also helped buy some extra time until eventually the module fails. TRY IT it will cost you nothing, except 2 to 3 hours of your time.
If that fails, then replace just the ETM, leave the tranny alone, and see what happens.
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'88 240, '92 745, '98 v70 John, Tampa Bay