Cisco WAN :: 877 - ADSL Drops When Launching E Mule

I have been using a Cisco 877 router at home for more than a year and I've always been quite happy with it, but I've stumbled upon a very strange issue after an IOS update. I was using IOS 15.0(1)M until last week, when I decided to configure the router as a VIN server (both for PPTP and L2TP/IPSEC) in order to be able to connect to my home network from outside; then I realized that IOS has a nasty bug on PPTP VPNs (it just ignores the ppp encrypt mppe auto instruction), so I updated it to the latest 15.1 release I could find, 15.1(3)T1. Everything worked.
 
Until I launched E mule with some heavy download, and the connection dropped. And didn't come up again, even after a clear interface ATM 0: only rebooting the router solved the problem. Which, upon launching E mule again, again happened. The problem seems to be related to opening lots of connections at the same time; "simple" heavy load (like downloading a big file at full speed) doesn't do any harm to the line, and configuring E mule to use a smaller number of connections and opening it more slowly seems to mitigate the problem, which anyway keeps happening after a while.
 
The strangest thing is, this is definitely related to the IOS version. It didn't happen before the upgrade, and I confirmed it stops happening if I reload the previous IOS on the router. Out of curiosity, I also tested some other IOS releases: 15.1(1)T, 15.0(1)M5 and even 12.4(24)T5. It always happens, only 15.0(1)M seems to prevent it... but it also seems to hate VPN encryption. And let's not even start talking about 15.1(4)M: I tried it and I wasn't ever able to successfully authenticate a VPN connection. Of course, when tried different IOS releases, the router's configuration always stayed the same. Here is it (stripped of personal details):
 
no service pad service timestamps debug date time local time service timestamps log date time local time service password-encryption!host name [Code] ....
   
How can I fix this without reloading the buggy IOS I was using before? If you need any debug information, feel free to ask and I'll provide it. This is the current show version:
 
Cisco IOS Software, C870 Software (C870-ADVIPSERVICESK9-M), Version 15.1(3)T1, RELEASE SOFTWARE (fc2)Technical Support [URL]

ROM: System Bootstrap, Version 12.3(8r)YI6, RELEASE SOFTWARE

Cisco877 up time is 34 minutes System returned to ROM by reload at 19:55:24 CEST Wed Jul 6 2011System restarted at 19:56:23 CEST Wed Jul 6 2011System image file is "flash:c870-advipservicesk9-mz.151-3.T1.bin"Last reload reason: Reload Command <snip> [Code]...

DSL: Training log buffer capability is not enabled. Also, any suggestion at fine-tuning the router; I can currently reach 7M/512K top speed, on a very stable line.