Constantly blinking Activity light

Well, you are very correct with the activity light blinking and the increased blinking except the fact that modem will encounter a problem in future...
The 5 different lights on your modem namely Power, Ethernet, DSL, Internet and Activity have their own indications and explanations.
1. The Activity Light indicates the amount of activity that is takin place on the computer on the Internet though the activity might blink even when you are not browsing. This can be explained by stating that the computer constantly keep sending signals  about it being connected to the internet and any other acknowledgements..

So, you've got nothing worry about unless the Power or the Ethernet or the DSL or the Internet light Blinks....
Cause they are the resultants of a fried modem(check for the adapter), faulty LAN connections, interrupted Dsl line and the account verification failure indications respectively.....