1999 Ford Puma revs hanging between gear changes

When driving along in my Puma, during gear changes [i.e. come off accelerator, dip clutch, select new gear, bring clutch back in] the revs don't drop and might even start to rise. Not until the clutch is brought in, or until the car comes to a complete standstill, do the revs drop back down again.

This is quite disconcerting. If you accelerate up to, say, 4000 rpm in any gear and then change up to the next gear, for a few seconds whilst in neutral and with no throttle, the engine will be racing away at 4000-plus rpm until the clutch is brought back in.

Normal tickover speed is also slightly raised. When all was working well, the car used to tick over at 875 rpm. It now ticks over at 1000 to 1100 rpm.

I've checked the throttle cable and it is not sticking. Inlet manifold has been changed recently, and while this seemed to solve the problem for a while (a month or so), it has now returned and is as bad as ever.

Might this be an idler valve, or throttle position sensor problem?