Volvo XC90 acting up uphill

You need to determine if the engine is missing or the trans is slipping. If the check engine light has come on, have the computer scanned. Most parts stores will do it for free. Since you are on a paved road and it does it even in the manual selection, that really leads me to believe it is an engine miss. At 79K, the trans isn't even broken in on this vehicle. However, it could be slipping. Have the computer scanned to confirm there are no trouble codes. Also, check to make sure the trans fluid is full and clean. If it is low, it could be jumping in and out of gear.

If it is surging while climbing a hill or under hard acceleration, I would suspect a bad turbo control solenoid. When they start acting up like that they normally will not post a fault code unless the solenoid has completed failed and had no boost pressure. But as Jacobandnickolas has stated, verify if it is the engine or transmission. Try to gunnit on a straight ahead and see if it misses that way, if it does then diffinately replace that solenoid. It is clipped on next to the air cleaner box with 3 hoses. Just match the color codes, green, red, and yellow. If the engine light flashes then you have a misfire and should have a code posted in the engine control module. Keep us posted.