Changeable basal insulin rates

I've been diabetic for 28 years now and have been on a pump now for about 9 months. I find that I can be reasonably stable for about a week on one basal rate and then something causes my glucose levels to go right up (19 without any problem) and I find I have to keep raising my basal rate until I find a new level at which I'm stable.
A week or so later my glucose level will be low and then I'll have to keep dropping my basal rate back down again until it stabilises again. I find my basal rate can vary between about 15 units a day to 22 units a day.
This has always been the case for me so its not just because I'm on a pump.
My lifestyle is pretty predictable particularly on work days - I work 5 days a week. I have the same breakfast every day and then cycle 5 miles to work. I typically work about 9 hours a day mainly behind a desk. I have a lunch of sandwiches made by me (very similar every day). I then cycle home at about 6.00-7.00pm. My evening meal is mainly rice and is again very similar every day (a boring life, I know!)
I very rarely get stressed at work but when I do, I haven't noticed any change in the insulin requirements.
I'm hardly ever ill (never had a day off work in 25 years).
There may be some hang over from weekends where my exercise and food is less predictable and I might have a few drinks as well. As my routine is pretty much the same every working day, I think my basal insulin should be pretty much the same - every working day at least.
Does anyone else have similar changes in basal requirement...or can anyone think of anything that might cause the basal to change that I can predict.