Variable jitter with PPP Multilink

Hi,
I have two serial links (2 Mbps) bundled with PPP Multilink:

interface Multilink1
bandwidth 4096
ip address 192.168.110.13 255.255.255.252
ip route-cache flow
fair-queue
no cdp enable
ppp multilink
ppp multilink links minimum 1
ppp multilink group 1
!
interface Serial0/0/0
no ip address
encapsulation ppp
no ip mroute-cache
no fair-queue
ppp multilink
ppp multilink group 1
!
interface Serial0/1/0
no ip address
encapsulation ppp
no ip mroute-cache
no fair-queue
ppp multilink
ppp multilink group 1

and this works pretty well. If I try to configure "Link Fragmentation and
Interleaving", to give better "priority" to interactive applications (as
telnet or voice) as described
in:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt6/qcflfi.htm

inserting:

ppp multilink interleave
ppp multilink group 1
ppp multilink fragment delay 10

link starts to go very bad, especially ping RTT show an highly variable
behaviour (from a few of ms to many hundreds).

Any help?

Thanks
Mimmus