sometime very slow pageload ( long parse time)

I have a strange problem with both my zencart shops.
http://www.luxim.nl/index.php
http://stockphoto.mrbertdesign.nl

That I have both hosted on servage.com
They are installed in separate folders and I have separate databases for each shop.
Many times the pages load within <3 seconds that is sufficient fast for me.
But then at random times the load time dramatically increase.
I see sometime load times of 20 to-30+ seconds and then it is almost always the parse time that is long and the query time rarely goes above 1 sec.
Most times query time is <0.5 sec.

I have beens searching with my hosting provider and they cannot find a thing wrong in the server and now the suspect the script.
I tried new install of the script without customization or addon's I tried installing in a new database ,nothing seems to help

Is there any thing I can do to see where the slow loading is caused?
the version of the Zencart script I use is :
v1.3.7 [2007-04-06 09:07:40]

This problem is driving me a bit crazy:S

Thanks in advance
Is there any suggestion that can help me solve this?

Sounds like a hosting problem. Check the server load during the slow times.

When I renew all websites (2 webshops 2 non php sites) that are hosted on that serer right after each other like you can do with iwebtool.com I see the other sites on that server load normally. sometimes just one of theh shops is slow while the other is normal some other times just the two webshops are slow.
Many times all load OK:wacko: