Hi sammymac I am in a very similar boat to you. My numbers were also doubling until they hit 2100 and then in 4 days they were only 4900.... Barely even 1 double let alone 2.
Incidentally I have also had the light brown bleeding which has also stopped. My doctor has not come out and said he is worried but keeps ordering new tests... In the end I have declined the HCG as they are just making me upset and in the end don't tell you much. I also saw that website about numbers taking longer to double after 1200 which would fit my levels as well.
I have a viability scan on Tuesday so will let you know if all is ok with my bub. Good luck to you xx