Valve stuck again...

Thanks for all the replies! Let me try to answer some questions. The first time it happened it was ex #7, second time in #7. This time it is intake #5. The heads were fitted with bigger valves and ported by what i thought was a reputable shop but with this being the third time i definatley wont be going back to them.

Also piston to valve has miles of clearance (over .250 IIRC) but i will do some investigating when i have the money. Moving this weekend so ill be busy with the new house but i do know the previous two times the stem seized in the guides at low rpm. This time it happened about two seconds after winding it up in first (quick run to 6300 rpm) possibly couldve floated the valves and touched one? But it wasnt instantaneous. Like i said i shifted to third and started cruising away before it stuck.

I will post more pics and specs when i tear it down i was just looking for pointers on what to look for. Thanks again for all the replies! I will update with more pics and clearances as soon as i can