Cisco WAN :: IP SLA ICMP-Echo On 3750?

Have a very peculiar issue with IP SLA. Firstly, the architecture.
 
1) There are two sites - A & B. Both have their own internet connection.

2) Sites A & B are connected via MPLS.

3) Both sites have the below topology.
 
3750 CORE --> FIREWALL -->ROUTER ---> INTERNET
 
4) 3750 has a Default route pointing to firewall .

5) MPLS router is connected to 3750. A default information is originated via BGP to MPLS at each location. So that default route is learnt as a backup path from any location if it has to lose its local internet.

6) IP SLA has been configured at each location to track the default route using icmp-echo to hit a public IP (i.e 4.2.2.2 as an example).
 
Issue?ICMP probes from Site-A via its local internet fails abruptly. I can reach the public IP mentioned above from my firewall pretty fine, but not from my 3750. Whenever i remove the tracking from the static default route & push in the plain default route without tracking, it works fine. Again, if i add the tracking back, it will work fine for an hour or so & then fails back again. To my bad, Site-B had recently gone offline due to some natural calamity. So, there is no other path for internet.
 
My config looks pretty simple
 
track 10 ip sla 1 reachability
!
ip sla 1
icmp-echo 4.2.2.2 source-ip 10.1.254.1
frequency 180
ip sla schedule 1 life forever start-time now
ip sla enable reaction-alerts
!
ip route 0.0.0.0 0.0.0.0 10.1.254.1 track 10
 
I am running IOS version 12.2(53)SE2 (IPservices images).