Cold boot but won't warm boot

Question

I have a system with a 648 Max MSI-6585 Mainboard.
It works fine once its running
BUT
it won't warm boot.
If I try to reboot or sometimes even power it up normally
I just get an initial beep and not the final POST beep
The D-Bracket lights say it is at the floppy seek stage.
To boot it I can:
(a) Turn the power off at the wall
(b) disconnect the ATX power from the mainboard
(c) clear the CMOS
(a) usually works, (b) generally works if (a) doesn't and if neither of them helps (c) always works.
If I write ANYTHING to CMOS (ie change BIOS settings or even if Windows sets the clock) then I HAVE to clear the CMOS.
This problem developed BEFORE I flashed the BIOS but I ave now flashed it (twice) to v 1.5.  No problems with the flashing, and I'm experienced at flashing bios's.
Any ideas?
Stats:
Using a P4 2.4 GHz CPU
With 512 Megs PC3200 DDR RAM
Hercules ATI All-in-Wonder Radeon 7500
Hercules Firewire DV card.
120 Gig 7200 rpm ATA 100 WD HDD with 8 meb cache
Pioneer Slot-in DVD
Aopen 52x CD-RW
Realtek Ethernet Card.

Answer 1

Since the last post I tried the system with different PSU's, different ram and different video - different everything in effect.  Eventually I gave up and took the motherboard back for replacement.  The new board is HT ready but I have HT disabled as the CPU is not HT capable.
The new motherboard worked at first then started doing the same thing.
I notice that when the cmos is cleared it detects the CPU as a 1.8 and not a 2.4 GHz.  I have had some success in getting the system to reboot by changing some memory settings.  Even though I am using PC3200 DDR RAM it seems to boot more often if I set the RAM at 333 MHz instead.
I have also set the performance timing to safe and altered the CAS setting and another  timing (MT I think?)
The system rebooted with these a few times but has since reverted to lights on, no action mode, with the D Bracket lights stuck at 'floppy initialisation'or sometimes 'memory'.
I am at my wits end with this problem.