Violent shaking only in first and when warm

got a 98 v6 mustang that takes off in first just fine when cold, but drive it for 15-20 mins and it starts to shake violently only when taking off in first. it did this before and after replacing the clutch and quadrant. flywheel was resurfaced, completely replaced clutch(f1 stage 1 racing clutch), replaced stock quadrant with a upr quick release quadrant and firewall adjuster. the only things that havent been replaced are the clutch cable and fork. i have adjusted the clutch up and down this has not helped. i was thinking it was mounts but my mechanic would have told me if they were bad so i dont think that is it. the only way to get it to go away is to rev it up to 1500-2000 rpms and let it out noramlly or to launch it. any ideas?