<VV> powerglide speed

[Michael Kovacs]

   While legally driving my '66 140 PG, I noticed several times with the PG locked in low, the engine would only rev up to about 4400 before it started misfire/break up. This happened 3 times. This equates to about 55-60 MPH. 
 Is this due to any physical restriction built into the system? The engine was pulling very strongly and accelerating just fine at this point.
 It is a strong rebuilt engine with true compression at 9.25:1 and a mild cam. I just realized this week that the sparkplugs were not changed since the rebuild about 30,0000 miles ago. If one starts to misfire at this rpm, could it cause the problem?
 Recommendation for spark plugs please...