sounds like clutch slipping. the more load you put on it that will reach a point where its can't handle it and it slips hence on boost. more boost = more load When the clutch slips that disengages the engine to a degree and allows the crank to spin faster hence the revs increasing.
id price new syncros if i were you and change them while you have them out. they might look ok but could be worn. changed my 5th gear and the syncro looked fine, swapped it out and what a difference