Suddenly getting Server error: Premature end of script headers

Hi all

Getting the following type of the same error

Hi all

Getting the following error from the error logs when a 500 error comes up

:41:30 2012] [error] [client 81.129.231.62] Premature end of script headers: php, referer: http://www.cityaquaticscardiff.co.uk/aquariums?gclid=CK7LvbKGvK0CFVQLfAoduBR4AA

[Fri Jan 06 18:38:47 2012] [error] [client 90.193.49.90] Premature end of script headers: php, referer: http://www.google.co.uk/aclk?sa=l&ai=CYXZLtj8HT42_FYSchQer-LQk_9y_0AGXsMGNK4ro3gcQBiDHmPgFKAhQv9DHnvz##########AWC71rmD0AqgAaHkhusDyAEBqQKHqJpF1du5PqoEGU_Q7VRBWrGg6VSDEAvR6IQ-nj7NnEyYoCKABZBO&num=7&sig=AOD64_1tKLD08zgPF_fKn51Xl4EgWmBS2w&ved=0CCsQ0Qw&adurl=http://www.cityaquaticscardiff.co.uk/aquariums&rct=j&q=fish%20tanks

[Fri Jan 06 18:19:20 2012] [error] [client 86.10.5.204] Premature end of script headers: php, referer: http://www.cityaquaticscardiff.co.uk/marine-products/hydor-wave

I have read a few forums\threads about this but the one piece of information i seem to be missing comparted to everyone else is im missing between headers: "insert something here i.e. easypopulate etc" php

any particular reason nothing is appearing there that would give me a better indication what is failing. The 500 error appears sparatically on the front end but i cannot get into the admin section at all - keeps throwing up the 500 error. this has also only been happing today

https://www.zen-cart.com/tutorials/index.php?article=63