Over the past year I have worked out lot of bugs on my SB stroker . The one last issue that has me puzzled is that after driving for 20 or 30 minutes the engine barely turns over. Here are the details:

Motor has less than 500 miles on a brand new build
Everything is new from the builder and ran on a dyno 458hp/497tq
Timing is as follows. 18 deg initial, 16 cent (recommended by prof builder.34 deg all in at 3100 rpm Distributor was sent out and put on dwell machine.
Cam is Comp Thumper Roller. Running temp with new aluminum downflow rad is around 175-180 deg. Engine idles around 950rpm with no issues. When I take the car out it runs great ( has a little surge) but overall runs ad responds great. However when I return to the garage and shut it off and attempt to reatart the starter ( a brand new Powermaster good to 13.5 comp ratio) mine is 9.5:1 , drags and barely turns the engine over. After waiting for 20 to 30 minutes it starts up without issue. Any suggestions as to a solution or remedy would be greatly appreciated.Could it be a timing issue? I might mention that one suggestion was to dial back the timing to 13 degrees initial and see how that works.