When it's absolutely clear outside, can you look at the HT1100 WebGUI and let us know what your Satellite Recieve Signal Strength is under System Status? It sounds like you may have an alignment issue that has slowly developed.
Transmit is much more sensitive to even slight misalignment. Can have good receive signal strength but off just enough to cause transmit problems. Again, could also be the transmit side of the radio is going bad. I'm sure Amanda will be back to help on Monday.
Alignment aside, it's good to check to see if the dish is fully in the clear from surrounding foliage:
Many things can cause these errors, to include dish alignment and poor cabling, which would require a skilled technician to remedy. However, it's also possible that a quick inspection of the dish can show where the problem is.
Tree limbs around the dish that appear to be out of the way may actually be reflecting stray signals back onto the dish. These then cause standing waves that the modem interprets as transmission errors.
Generally, the dish should be clear of any foliage within 45 degrees in any direction. Pay particular attention to any branches that may be blowing into that area, which could possibly cause intermittent problems.
In the fall and winter you might also want to inspect the dish to see if there are any fallen leaves or ice stuck to it. These will reduce the reflective nature of the dish, possibly causing additional transmission (and reception) errors.
As well, cables and connections may become brittle or loose with frequent weather changes. So you may also want to see if the connectors are finger-tight and the cabling isn't cracked or weathered to the point where coax shield wire is showing through.
Some error codes (e.g., 12.1.19) stem from the modem's sensitivity to power levels. You may want to inspect the routing of the power cable to make sure the DIN plug in the back of the modem is firmly seated, as well as at the wall and at the power brick.
If those are fine, make sure that the modem's power cord is plugged directly into a wall socket and the wall socket is on a circuit intended for lights. Use on the same curcuit as heavy-use items (e.g., air conditioners/heaters, washers/dryers, ovens, etc.) will cause power drops that may contribute to modem errors. Powering the modem though a surge protector may likewise cause power levels to drop and is therefore not recommended.