What should I do, the LNB is still under warranty, should / will the manufacturer do anything about this?
The LNB should be stable over a vast temperature range.
the following is for a Cal Amp LNB from their web site
Specifications*
Input Frequency 3.4 to 4.2 GHz
Output Frequency 950 - 1750 MHz
Noise Temperature 20°K to 35°K @ 25°C
Gain 63 dB Typical, 60 dB Min
Gain Flatness ± 2.0 dB Max
Image Rejection 55 dBc Min
Output VSWR 2.0:1 Typical, 75 ohm
1-dB Compression Point +10 dBm Min
3rd Order Intercept Point +20 dBm Min
LO Frequency Stability 5.15 GHz, ± 125 KHz Max
@ -40°C to +60°C
Phase Noise -69 dBc/Hz @ 100 Hz
-85 dBc/Hz @ 1 KHz
-88 dBc/Hz @ 10 KHz
-92 dBc/Hz @ 100 KHz
-105 dBc/Hz @ 1 MHz
DC Input +15 to +24 VDC
Current 275 mA Typ, 300 mA Max
Operating Temperature
-40°C to +60°C
Input Interface
Flange, WR 229G
Output Interface 75 Ohm, Type "F" Female
Finish Powder Coat
Physical Size 3.86 x 3.0 x 7.0 in
Weight 20 oz
Note the temperature range of -40°C to +60°C with a LO Frequency Stability 5.15 GHz, ± 125 KHz Max over that temperature range. Your might not be rated for quite the stability of the above example, but it should not drift 13Mhz. The manufacturer should repalce it with no questions for a drift of that much.