After a weekend of heavy printing (visit the 3D printed nudes blog to see the successful prints) I suddenly found I couldn’t print properly any more.
Intermittent gaps in the layers were the hallmark of filament jamming in the nozzle. The fan on my E3D hot end was still spinning but was rather erratic and much quieter than usual. I should point out to @Sanjay_Mortimer that this is not the fan that was supplied with my v4 hotend which died last year, but a replacement I bought from Maplin.
I disassembled the fan, removing the sticker and a rubber sealing plug and then placed a drop of light machine oil on the axle. After moving the axle back and forth a few times to get oil into the bearing, I switched back on again and the fan was back up to full power.
I had this exact issue, on the same equipment. The bearing in the fan had failed.
I would start a print it would make an odd run up, be fine, hours later heat up and slow down due to wobble.
Its interesting how often lubrication plays into our stuff. Making a consumer device that wouldn’t need even an occasional lubrication schedule is a non-trivial problem.
I also replaced the fan with the rather high-end Evercool at £3 
Will keep an eye out for this issue!
I had my fan fail twice now. I switched to a 40mm fan. No problems since.
Cooling is the #1 most common problem I see with customer getting jamming issues on our E3D hotends. We’ve upgraded fans a few times since v4, and now run a fan with really nice 2 ball japanese made bearings which has made a big difference.
Some exacerbating factors tend to be things like heated chambers which can cause lubrication to gum up, also a lack of space for air to get in/out.