After a year of intermittent slow downs, most pages loading in under a second then a few taking several I have finally got the problem licked!
I went through all the usual steps, ensuing eAccelerator was installed and enabled in server settings, optimised MYSQL etc, etc. The other day we realised the problem was with eAccelerator. Just make sure that the eAccelerator cache folder is owned by APACHE not ROOT! once this change was made all page load times are around 0.1 seconds If the folder is owned by ROOT the accelerator can't write to it and although it appears to be working in SMF and even reports itself to be functioning correctly it isn't.
Gerry