Rapid typing in Index or Search field causes help file to freeze

My company has been using Robohelp 7 for quite some time, in support of a product built in VB6 (don't laugh, it still works). We're using Microsoft HTML output. RH7 gave us a few difficulties at the time of our last release - having transferred the licence to a new machine, we couldn't get the help to compile - but we got through them by reinstating the old machine, and we've had no indication that users have had any problems.

In the meantime, however, I have had some problems, in that typing too quickly in the text box on the Index and Search tabs causes the help file to hang. It doesn't come back, and you have to kill it in task manager, which takes the host app with it.

Our backup RH7 machine is no more, so I've been wrestling with RH7 on a new one again, and it's been such a nuisance that I resorted to trialling the RH 2015 version. This compiles perfectly, right out of the box, so I'm ready to ask the Finance department for the money for it. However, even though the file was made seamlessly, I still get the hanging. It's not just my machine, I've embedded the .chm into our build and had someone else recreate it as well.

So it's not machine specific, and it's not even RH version specific. There must be something in the source files somewhere that it doesn't like? I'm pretty much a noob at Robohelp since I only took it on in the last year, so any suggestions for what might be causing this - however basic - would be welcome.