[70Sixter]

Does not sound like clutch to me if only symptom in the shifter. At steady speed of accelerating?

Any change in clutch pedal feel or when it engages/disengages the clutch? Spongy pedal? Gear grinding going into gear or shifting? Bled system it recently? Slipping in lower gears with medium to heavy gas pedal pressure?

If not, could be the flywheel. Maybe the transmission. Check oil level.