hesitation/surging issue when above 50mph

[kdswatosh]

I have a Volvo s70 which has been a pain in the ... for the last few weeks.
We experience surging above 50mphs when going a constant speed.

The car does a hesitation/surge off and on. Brought it to Volvo dealership and after they diagnosed it as a ETM software problem they sent us out the door. The next day...still surging. WHAT THE HECK?!

Went back today and they now say it is a Solenoid Valve or turbo control valve problem...only after a $120 diagnostic test for this round as well as $120 diagnostic for the prior visit. Should we trust them or not? Do you think it could be the Solenoid Valve or still an ETM problem.

If you think it is the Solenoid Valve, can you give me some directions on how to replace it myself and avoid paying the dealership another $130 in labor.

Thanks fo the help!
A 1999 Volvo s70 unhappy owner!