Timing belts are usually good or dead. Not failing slowly. Your problem could be a timing belt that has jumped a tooth, a very rare experience on the BBRD and never in mine.

Not knowing which model and year Volvo you have, this must be considered a SWAG. If the engine performance changes as it heats, there's a chance that the emgine temp sender to the ECU has failed.

More posters will have more ideas than mine.

Good Luck,

Bob

:>)