FubarI33t

a few weeks ago, i blew my #6 exhaust valve, got all the fixed, now i got a small problem. she drives fine, but after about 10 mins of driving, the rpm gauge will go nuts, and the engine will start stalling, i keep driv ing it, and it will fade off after a few mins, but it happends Every time i drive it, it never fails, if i drop her in N and coast the engine will die, but the rpm gauge still go crazy with the engine dead, but she will start back up. and still acts up, any ideas? it's driving me crazy. oh, and PS. i got me a 85 V6 4 speed Fireo, ima get it goin, and drive it while the Z gets some lovin.