plustek: Thread: Scanning under slightest load ruins image data (PRIMAX 4800 Direct)

Hi all!

Today I got a Primax 4800/Direct (parallel port) from the 
garage-sale.  I installed Sane and the Plustek driver to 
get it going. Everything works fine (really fine ;-) until 
the system load increases (e.g. due to another program like 
mozilla displaying an animated-gif). The resulting image 
then has it's ground-colors splitted and randomly shifted 
(together with complete dropouts sometimes) In other words, 
data transmission seems to get out of step. The effect gets 
worse with higher resolution (max. 300dpi for the Primax
scanner)

I tried to renice the scan program (xscanimage) to -20 
(under root) -> no effect, when another program steals 
some CPU-cycles. Obviously "niceness" doesn't help 
because a lot of the processing happens in kernel 
space through the pt_drv module. I have no idea what 
I should do now. Shouldn't the kernel buffer incoming 
data while it is doing somethin else? Should I increase
this buffer somehow? Or do we have an overun in the 
UART buffers? Any hints appreciated ...  


I use: 

Linux 2.4.17
Sane 1.0.8
plustek-module-0_42_9.tar.gz

/etc/modules.conf:
---------------------------------------------------------
alias char-major-40 pt_drv
pre-install pt_drv modprobe -k parport
options pt_drv  lampoff=180  warmup=15  port=0x378  
                lOffonEnd=0 mov=2 slowIO=0 forceMode=2
---------------------------------------------------------

`dmesg` on driver startup:
---------------------------------------------------------
pt_drv : driver version 0.42-10
parport0: PC-style at 0x378 (0x778) [PCSPP(,...)]
pt_drv0: Primax 4800 Direct found on port 0x0378
pt_drv0: Lamp-Timer set to 180 seconds.
pt_drv0: WarmUp period set to 15 seconds.
pt_drv0: Lamp untouched on driver unload.
---------------------------------------------------------

`lsmod`:
---------------------------------------------------------
Module                  Size  Used by
pt_drv                112128   0  (autoclean)
parport_pc             12336   1  (autoclean)
parport                14528   1  (autoclean) [pt_drv parport_pc]
agpgart                15760   3  (autoclean)
NVdriver              821200  14  (autoclean)
vmnet                  18336   2 
vmmon                  18400   0  (unused)
3c59x                  24896   1  (autoclean)
ntfs                   48176   2  (autoclean)
nls_iso8859-1           2848   3  (autoclean)
nls_cp437               4352   1  (autoclean)
msdos                   4848   1  (autoclean)
fat                    29888   0  (autoclean) [msdos]
---------------------------------------------------------


Thx a lot,
Peter