i am a bit late in replying and have only read a few of the replies but I wanted to tell you that this is exactly what I experienced when I first purchased my super rocket, I duly treated it as new when I first got it keeping the speed on the low side for 500 miles and building up the speed gradually, during which time I increased the speed (revs) for short periods then reducing down to what I call running in speeds until I was happy that there was no noticeable problems.
When I went over 60 mph for any length of time the engine would start to tighten up and I pulled the clutch in before the engine actually seized, I then ran it in so to speak for a little longer but the same thing kept happening for any lengthy duration over 60mph, so after about the 5th time I took the bull by the horns and took the top end off, the pistons had tightened up in the bores and subsequently the rings grooves had pinched the rings.
I found out when apart that the pistons were 80 though oversize and had been supplied by SRM who also did the rebore now they reckoned that due to the make up of the pistons the clearance needed in the bore was less than BSA would recommend, I took the barrels along to our local BSA dealer (well he was in the 60,s and 70,s and is still trading) who measured them and declared them to be to tight a tolerance regardless of what SRM said. So after finding out that SRM had sold the remaining stock of 80 thou oversize pistons to MCA Hammond?s obtained a set from them and rehoned the bore to what they said was the correct tolerance, I duly fitted the parts back on rode it steady for a couple of hundred miles and have had no problem since, and ask any of the lads how I mistreat my super Rocket (70mph from Dover to Swindon continuously with only one stop and the last 6 miles between J15 to J16 M4 at 85 to 90mph)
So it would be worthwhile to take the top end of and check the pistons and bore even if for peace of mind.
Sorry for the book
Richard