Greg,
After rechecking the exact LO frequency of my TX converter and adjusting the DOPPLER.SQF file, I found I was pretty much on frequency last night. I am still doing some empirical testing, by slowly turning down the power, and tweaking the TX frequency in 1 KHz increments while talking. That has greatly reduced the dropout times compared with some other stations. Right now, it looks as if being slightly higher than the nominal uplink, 2-3 KHz, works better for me. Of course, that depends on how well everything in your station is calibrated.
Initially, try jumps of 5 KHz, up and down. If you are nearly centered, it won't make much difference. However, if you are significantly off frequency but still in the passband, one of those jumps will take you out entirely, and the other will make a great improvement. That is what I found. Once you find the right direction to go, try another 5 KHz. If it deteriorates, go back, and fiddle with smaller steps.
Also, as VE3NPC commented, the downlink may be slightly higher. I receive 435.300 with the discriminator centered, but I find this downlink looks better at 435.1515. No question you have to hit the bird a bit harder for a full quieting signal. Lots of fun, though.
Alan WA4SCA