doesn't like 50mph

I'm having a problem that my local shop doesn't know what it is.
Not every day, but about every other day now when I get it up to 40, 45,50, 55 mph it shakes and jerks through all those speeds. It doesn't do it when I go slower or faster. There is no loss of power just a shaking feels like under the drivers seat. I just had the moter mounts replaced. Has anyone else had this problem?

My 78 camaro does the same exact thing. Only at those speeds, only when going at a constant velocity(no speed up or slow down).

No idea why its doing it though, sorry :disappoin

have you had your tires balanced? it sounds like a vibration harmonic, like something is out of balance, could be tires, driveshaft.

also i dont know if this is your case, but on third gen camaros, it is not uncommon for the lockup converter to cycle (lock and unlock) at those speeds while the electric fan is on. apparently its something about the vibrations of the fan combine with the vibration of the engine to create a harmonic that sounds like detonation to the knock sensor, so it retards timing, lowering engine speed, and causing the converter to unlock, then because the engine speed changed the harmonic is gone, so the knock sensor dosnt hear a detonation, so it starts running smooth, then the converter locks up, and brings it back into harmonics, causing the knock sensor to hear the.... oh well you get the idea, its just a circle of events that keeps going. you might have a situation like that.

I have the same problem with my 89 camaro but my check engine light blinks when it does it, and it doesn't do it all the time either. It usually happens when it is really humid out...how do you fix it? replace the knock sensor?