You haven't mentioned the vendor or model. It sounds like a bug with your device, or it's taking too long to respond because it has so many interfaces. (I had this problem with old Dell Powerconnect 5524 switches; they show hundreds of interfaces as if they were in a stack. I could scrape them at 1 minute intervals, but not at 15 second intervals. The newer Powerconnect N1548 are much faster)
What happens if you use snmpbulkwalk to walk the IF MIB of the stack? How long does it take to complete?
You could also try
tcpdump -i eth0 -nn udp port 161 and host my_stack
while you're doing a scrape, to see if data is trickling back as it runs.