Spend a little time going over the intake / exhaust manifold mounting bolts. It sounds like you have vacuum leak at the intake. With both manifolds bolted together, its hard to believe you had a exhaust leak with out also having a intake leak. You might try taking some weight off the manifolds and loosen the bolts holding the intake to the exhaust manifold. Then tighten the manifolds to the head. If you did not get the manifolds to head surface machined flat with both bolted together, it can be tough to get them to seal right with out some fussing around. Also be sure the alignment rings are correctly located if you are using them.
Don't fire it back up till you are sure about the leaks, and as you are aware already, don't keep running it for short burst like this. Once it starts, let it go. Adjust the timing while running and watch the temp gauge for excessive heat or quick heating. RPM is key to cam life, its the crank throwing oil on to the cam lobes, and low rpm will not get it done!
