It could be other things such as a vacuum leak, but one thing to check
is a free unofficial recall.  There is a "service campaign" for the
electronic throttle system (ETS) and electronic throttle module (ETM)
where they actually load a new program that ignores errors by the ETM.  
If they need to replace the ETM, it covers that also.   It is not
techically a recall, but "VSC 155 service campaign".   They fix the
ETM/ETS for 10 years or 200,000 miles, whichever comes first. They also
reimburse people for repairs they have already paid for.  For more
information call Volvo Customer Care Center at 800-458-1552 or the ETM
HELP DESK at 866-637-3774.

Here is what the campaign covers:

"Electronic Throttle Module (ETM) failure may illuminate Electronic
Throttle System (ETS) light or MIL. Symptoms may include, uneven engine
idle, and/or "limp home mode" that could restrict vehicle speed to 10-30
miles per hour. Faults of the ETM are stored as DTCs in the ECM. If OBD
II detects a fault that might increase emissions, the MIL is
illuminated. Remedy: A software upgrade will be performed at the next
service interval, and ETM warranty extended to 10 years or 200,000
miles, whichever comes first."

It basically covers S70, V70, V70 XC, C70, S80, and S60 from 1999 to
2002.

For more information check page 2 of:

http://www.epa.gov/otaq/cert/recall/2006recallreport4.pdf

I have a '01 V70 XC.  My symptoms were:

1) The idle stumbled

2) The cruise control would reset and wouldn't work until you shut off
the ignition and turned it back on.

3) Occasional dead throttle when starting from a stop.

It fixed all three.  It runs like new and all they did was the software
update.