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The clutch disc is sticking to the flywheel or onto the splines of the trans input shaft. If the front trans seal leaks a very slight amount it will make the disc sticky and vacuum will hold it to the flywheel, like a wet water glass on a smooth table. The other side of the story is the disc splines get so dry the disc will not slide the slight amount it needs to release when the pressure plate is pulled away. Either way the trans must come out. Volvo makes a super duper high temp slippery forever $40.00 a can grease for the clutch splines. Any other grease will dry out and harded up and become glue on the splines. If the clutch was replaced than no grease or glue grease was most likely used on the splines. If the trans shifts ok with the engine off the trans is ok. Don't listen to your "shoot from the hip" mechanic. If it is sticky splines I have gotten a long tube with penetrating oil and sprayed a small amount onto the splines. This has worked miracles if the sticky splines are the problem, although this is only a temp. fix.