There are quite a few things that will create position errors. For example there is marginal accuracy for civilian use of government satellites but to be 100 yards off is more than I have ever experienced personally. There is also a big influence from the sun. There is a great article in National Geographic where this quote comes from;
"A flustered ionosphere deranges GPS signals as well, resulting in positioning errors that can be more than 150 feet."
Here is the link to the full article;
The article is kinda technical but very interesting.
As for the LGC 2000, there is no need to change the antenna as long as it is still locking onto satellites and giving you a position lock. The new LGC 4000 updates your position 5 times a second whereas the LGC 2000 updates your position once every 5 seconds. The problem is your LCX 26c HD does not have the processing power to utilize the LGC 4000 optimally. The older units are not as accurate as the newer ones like the HDS Gen 2's.