Hi Scooby,
Gosh it looks like we are in exactly the same situation hun.
I had BFP on the 7th of Sept.....HCG was 156 Prog 95
1 week later HCG had only risen to 400.
My clinic said that it's not really good news but not totally over either,they said that HCG levels can vary widely and that the only true way of knowing if you are having a healthy pregnancy is at the 8 weeks scan.
I have looked up heaps of websites about this stuff and to be honest from what I've read it's not an exact science....so hang in there love....I totally understand how hard it is though.
I have my 3rd BT this fri and I suppose that will tell for sure wether things are going ok...I am trying so hard to think positively.I still have a bit of morning sickness,though not as bad as the week before the BFP...other than that no syptoms.I am taking heaps of progesterone 3 x 3 times a day,so presume that the drugs may be keeping things at bay.
God it's all so hard isn't it? Well fingers crossed for you love...hope the levels keep going up,at least they're not going down!Wishing you the very best of luck.

Hugs,
Claire