The white powdery stuff around the inside of the feed-horn shouldn't make a lot of difference, but if the probes are corroded, that might make a huge difference.
Can you get detailed pictures of the v-shaped probe at the mouth, and more importantly the probe at the back used for C-Band?
Second question. I assume that the servo motor is working properly. If it isn't the skew might be adjusting right, and can kill the signal.
Try G1 transponder 11 and 12 and make sure the polarity changes properly. 11 is Horizontal, and 12 is vertical. If both channels don't come in without sparklies, the skew may be off.
Also, what LNB are you using? Brand, and temp?
Let us know and we'll try and help.