Wiper travel

[merc uni]

I'm having a rather baffeling problem with my wipers on my 62. They work fine for about half an hour of continuous use and then gradually increase their arc until the blade starts to slap against the lower window seal. There is at least 1" of clearence on start up, so the travel is increasing as time passes. The bushings look OK, and there are stops built into the stub mounts.so I am at a bit of a loss. Anyone run into this before ?