Can't access recovery

A friend has given me his Desire S to fix what I originally thought was just a stuck power button that prevented it from starting up. But the button is now fixes and I've been trying the last few days to get it to start up.

What I have tried so far is to enter Recovery mode, but upon choosing it, the phone would just go to the HTC logo screen, fibrate once and then fibrate agian after a second or two for a few times and the screen would go blank. The same happens with normal bootup. It also seems that the phone is still running with the blank screen because I have to pull the battery to startup again.

I then tried to use the latest RUU (Android 4.0) from HTC but the phone is not recognized by the computer (I think it is because it didn't startup enough to be able to present itself to the computer). So I unlocked it and used fastboot (from Ubuntu as OS) to flash the recovery (recovery_signed.img) and boot (boot_signed.img) from the RUU. With this recovery and normal startup would remain at the HTC screen and wouldn't go blank as before - still fibrates a few times.

I also tried to rename the rom.zip to PG88IMG.zip and putting it on a SD and the bootloader does detect it, but presents the normal boot screen afterwards (I think it complains about it being incorrect - the display is too quick for me to see the error).

Now I also have a Desire S and made a backup of it before I installed a custom ROM. So I also tried the recovery.img that I have from this backup, but then recovery is stuck on the blank screen again.

Is there anything that I did wrong, missed or may not have tried yet? And how can I go about to fix it?

PHONE DETAILS (from bootloader screen)
*** UNLOCKED ***
SAGA PVT SHIP S-ON RL
HBOOT-2.00.0002
RADIO-3822.10.08.04_M
eMMC-boot

Ok, didn't know that I needed S-OFF for PG88IMG - thanks for pointing that out . I did flash the recovery (paragraph 3), it is just named `recovery_signed.img` and not 'recovery.img` - I assume that there isn't a difference between the two?

Do I open the RUU while in fastboot on the phone (after relocking) or do I run it from a fastboot command?

(On Ubuntu fastboot is installed as a program so I can call it after I CD to the extracted rom folder and I assume it is working since I didn't get any errors)

Ok, relocked it and went into fastboot mode and then opened the RUU, but was not recognized. Then followed this (droid-now.com/2013/10/fix-working-fastboot-drivers-windows-8-1-x64/) since I used Win 8.1 for the RUU and it flashed successfully.

But it is still stuck on the HTC screen - gave it 30 minutes for the first boot after flash. Recovery is also still stuck, so I reflashed and same issue.

Radio is now 3831.19.00.110