Transmission Grinding: Am I doing something wrong?

I've been able to replicate the problem without fail every time since I noticed the problem. I am 6' tall and no winter mats, so its not that I'm not depressing the clutch all the way. I will give you exactly what I do when I face this grinding issue.

- Approach highway entrance, downshift into 2nd gear and take turn at roughly 15 - 20 mph.
- Begin acceleration in 2nd and shift to 3rd
- After I get to ~ 55 - 65 mph (roughly 4,000 - 4,200 rpm) I depress the clutch ALL THE WAY to the floor, I've even held it on the floor for a good 3 - 5 seconds to make sure that the clutch has fully disengaged.
- Pull shift knob out of 3rd (or 4th depending on where I was) and put it into 6th, at that time I hear grinding.

I'm 21 years old and have been driving manual on and off for the duration of my driving experience.

Alright, I've done some more tests and here are my results:

A 'double clutch' shift (shifting to neutral and releasing the clutch then shifting to sixth) does not cause the problem (I will try again later today on my way to work).

However that did sort of cause me to try another thing.

While accelerating to ~60mph in 3rd gear, with the clutch fully depressed, shifting into 6th causes grinding, once in 6th, I pushed the clutch down again, shifted out and shifted back in (releasing the clutch while in neutral), no grinding.

While driving at about 30 - 35mph in 3rd gear (usually I'd cruise in 4th but this was for experimental purposes) I fully depress the clutch and put the transmission into 6th, without re-engaging the clutch - no grinding.

My final test was to, while traveling at ~30 mph, push down the clutch, and bring the engine's rpm to 4,000 - then immediately put the car in 6th gear (without re-engaging the clutch), still no grinding.

Therefore, to me, it seems that the transmission only grinds when shifting from 3rd (or 4th) straight to 6th while under load.

Thoughts?

Got an answer today, here's the TechLine transcript:

*** NOTES 12/14/2010 05:47 AM clarify Action Type: Manager review
*** Performed by contact:
*** This is a Request for Assistance ***

Problem Description :
Customer states that they cant shift from third gear (with rpm's high) into sixth gear. Fourth and fifth gear go in ok. Third to sixth shift grinds.

Diagnostics Performed :
Test drove and verified concern. Is this normal?


*** PHONE LOG 12/14/2010 06:09 AM Pacific Daylight Time RHicks Action Type:Incoming call
Advice 1:
I believe this is normal... I'm driving one now and it does the same...
it is recommenced to shift all the way through gear pattern normally
- this reduces wear on blocking rings/springs and syncros