intermittent engine vibration at full song

[rutroracing]

I have a vintage racing 914 with a H6 2.7 (made from and older H6 2.0 case).
In the process of "driving in anger" last season I spun and couldn't get to the clutch in time and spun the engine backwards at about 60 MPH (guessing). I know, shame on me, right?
I went out in the next session and all felt OK until half way through the 15 min session and at about 5500 - 6000 RPM the engine developed a vibration. I shut it down and went back to the pits. I checked the plugs and wires for connection and tightness but all was well there. I took it out for the next session and the first time I got to about the same RPM in 4th and 5th gear the vibration returned. Backed off and the vibration stopped. Got back up to speed and about 2 laps later the same vibration returned.
It's past time to try to figure it out now that the season is upon us again but I'm not sure where to start looking.
It still seems to have good power, no noticeable decline.
I haven't done a leak down on it yet but plan to this weekend. I have some other ideas but admittedly I would just be guessing and end up replacing things that don't need to be replaced. The engine is a few seasons old now, it's not a grenade engine, and has been really reliable so far. Probably could use refreshing now anyway.
I could sure use a list of things to investigate /rule out. Any help would be greatly appreciated.
Thanks,
Bueller? Bueller?