Wiper problems

[kslett]

Sorry if this has been covered before...
On occasions my wipers will stop halfway through their sweep. If I get out and give them a nudge they'll finish the sweep, but that's not much use to me in the rain on the motorway (yeah I know someone will say I shouldn't take the Chim out in those conditions, but I'm too poor to afford another car!). It seems to happen most if they're on intermittent wipe and I turn them off; not seeming to happen on constant cycle. Thanks in advance for any advice.