DHCP server answers with delay

[Denis Taranushin]

Hello, All

We have trouble with DHCP. namely the DHCP server answers with delay.
Sometimes the Server answers on 2th or 3th "Discover"
In this time a Client sends some "Discover"s and the server answers each 
of them.
So in final the Client receives 2-3 "Offers" of IP, and accepts the last 
one.

It makes impossible to work billing because the billing does session by 
the first "Discover" and the second and each next will be forbidden.

Why the server answers in 4 secs after the first "Discover"?

tcpdump on server:
1335	2014-04-02 11:04:41.173969	10.0.0.1	10.0.0.13	DHCP	378	DHCP 
Discover - Transaction ID 0x21fe4bf0

2344	2014-04-02 11:04:44.168945	10.0.0.1	10.0.0.13	DHCP	378	DHCP 
Discover - Transaction ID 0x21fe4bf0

2554	2014-04-02 11:04:45.171448	10.0.0.13	10.0.0.1	DHCP	369	DHCP Offer 
   - Transaction ID 0x21fe4bf0

2559	2014-04-02 11:04:45.196749	10.0.0.1	10.0.0.13	DHCP	402	DHCP Request 
  - Transaction ID 0x21fe4bf0

2562	2014-04-02 11:04:45.199567	10.0.0.13	10.0.0.1	DHCP	369	DHCP ACK 
   - Transaction ID 0x21fe4bf0

3813	2014-04-02 11:04:48.714916	10.0.0.1	10.0.0.13	DHCP	373	DHCP Inform 
   - Transaction ID 0x8b4c15cc

3819	2014-04-02 11:04:48.719283	10.0.0.13	10.0.0.247	DHCP	342	DHCP ACK 
     - Transaction ID 0x8b4c15cc


Unfortunately I can't provide the config because it is enormous.

Some ideas?