1st and 2nd gears don't like to go in place when warm

Hey all,
my 88 z24 is being a money pit again. when my car is running for about a half hour in extreme heat, I have to use all my body weight to shift into second and first or first to second. its as if the clutch and flywheel aren't exactly matched and I have to push or pull really hard to get it into gear. My uncle thinks its the clutch actuator or something like that... the arm that pushes the clutch into the flywheel. I say its going to cost a lot of money.
They grind a lot when I am at a dead stop usually and sometimes it just goes right into gear like a hot knife in butter. any ideas??

Thanks

the frame and fuel lines were fubar so I decided to sell/part it out. anyone suggest a good car for under 1500??

Cobra