Screw cutting feed keeps jamming

I have an old roundhead Colchester Student 6" lathe, I believe circa 1963.

I have just started to have a problem with the screw cutting. It starts to lock up when I am screw cutting. The lead screw is in reasonable condition and the half nut doesn't look too bad. Anybody have any ideas what might be causing the problem. I do a fair bit of screw cutting and it has only just started to jam.

Any help would be appreciated. Thanks in advance.

How do I insert a picture from my desktop? Or where do I create an album to get picture from?

Have a read of this to see how to add pictures

Graham, A bit more info please what's locking up? lead screw, half nuts or ???

Just to post this to be part of the record for searching help for others. I had a problem with a Clausing Colchester with the lead screw shear pin shearing while threading. Took a year of off and on again detective work to find the cause. Always threaded with the lathes motor (naturally), seemed to be irregular in happening. One time I tried to thread by pulling on the chuck around and found a tight spot. Marked the ways and the spot repeated when the lead screw got tight. Could not see under the rack so disassembly was called for. Found Someone had the apron off and had jacked it into place and mashed one rack tooth over partially. You could not feel it when turning the handwheel to advance it towards the head stock. When the lead screw was driving the apron the back lash was not be taken up and the gear would bind then shear the pin. Hard to find because I never turned it over by hand and it seemed random because of the different lengths of material threaded. Posted this in case it helps some one out.