Pogostick Continentals and Vibrating New Series

I'm using Continental Grand Prixs on my APB. These roll very nicely and are very light, but I've had my fair share of problems with them.

Firstly, one burst during my commute about 2 years ago. I wasn't going very fast, the inflation pressure was about perfect, I hadn't hit a pothole, and I wasn't carrying a load when the sidewall on the rear tyre blew out.

I was tempted to ditch them there and then, but I thought it was a once off and that it would be a mistake to write off any product, especially a tyre, based on one possibly freak occurrence so I bought a replacement and a spare.

Then late last year, when I put them on my Rigida Xplorer rims I got a pogo stick effect due the tyres not making a near perfect circle. It affected both front and back and made the bike unrideable.

I spent hours trying to get the bead to sit correctly on the rim, without success.

I partially solved the problem when I discovered that a medium width rim tape had been used when the wheels were built when a narrow tape would have been more appropriate.

The rim tape sat under the bead in places and not in others contributing to the inconsistent diameter of the inflated tyre.

What finally made the ride bearable again was the old trick of smearing washing up liquid on the inside of the rim wall, to reduce the friction between the bead and the rim. The bead is then encouraged to move more evenly outwards as the tyre is inflated.

Now the engineer in me did take into consideration that maybe the rims were at fault. I did previously have these tyres on standard Pashley wheels without a pogo problem.
However I also had Schwalbe City Jets on the Rigidas without a problem. It's difficult to say with certainty without an extensive experiment, but while the rims may be a contributing factor, they are unlikely to be the whole of the problem. I must conclude that the tyres are at least part, if not all of the problem.

What has this got to do with the New Series?

Well, on the moultonbicycle list on yahoogroups, there has been talk of a mystery vibration that occurs with Contis on the New Series, that kicks in at around 33km/h.

Well today, while the APB was on the work stand, I noticed the pogo effect again when I spun the wheels. And I noticed that the whole bike started to vibrate significantly at a certain speed. I used the bike computer to ascertain that this happened between approx 34 kph and 44kph. I repeated the the test several times with the same result. I also spun the wheels up to 70 kph, steadied the bike on the stand, and allowed them to slow down, thus ruling out that my hand pedalling motion was inducing any vibration.

the result was the same... Extensive vibration started at 44 kph and stopped around 33.

So are pogo stick contis a problem for others? Are they the reason for vibrating New Series phenomenon?

Anyhow, I have made a commitment to Contis for the time being having a pretty unworn pair on the APB, and a spare pair in the shed. So I'm pretty determined to make them work for me, instead of me working for them as seems to be the case at times!