[jhv48]

Since you have a hyudraulic clutch and you say that the problem involves both fifth and reverse, it does sound like a clutch release problem.

One more thing to try is bleed the slave cylinder and replace with Wilwood hi temp brake fluid. Your fluid could be old (with perhaps a little water in it) and boiling when you go hard causing a weak release. Then, when things cool off, all is back to normal.