Difficulty downshifting 915 transmission to 3rd

['78 SC]

Is this a symptom of worn synchro ring?

My 1983 911SC track car (DE use) has recently become difficult to downshift from 4th to 3rd. It feels as though something is blocking the last bit of shifter movement. All other shifts, including 2nd to 3rd upshift are fine. It does not grind when shifting. Occasionally after a shift, it pops out of 3rd gear when power is applied. Double-clutching 4th to 3rd usually works (I won't claim my technique is 100%).

Bushings and shifter adjustments have been checked. The fork in the transmission that guides the shift dongle is secure. Gear oil recently replaced. Drain plug magnet had a bit of "fuzz" but no chunks.


History: I've had the vehicle since March 2015. I have done no transmission work other than gear oil changes and checking the shift fork. Engine and transmission are from a '82 911SC installed by previous owner. Purportedly transmission was "rebuilt". This is probably when the differential was upgraded.

Transmission ID: 915/63 74D 1815
Differential: Guard LSD
Shifter: JWest RennShift
Shift linkage coupling: Stomski Precision shift coupler
gear oil: Swepco 210 (red stuff) recently changed. Previously used Swepco 201 (blue stuff)

Thanks in advance
Steve