I don't know if you also have a 5th gen but the motor on my '92 starts to slow a little after around 15-20 minutes of use - that's in around 70degree heavy rainfall. It's also acted up once or twice in the cold of winter b/c of ice building up at the base of the windsheild.

I don't know if it's just getting hot enough that it can't go at normal speed or if it's protecting itself from damage by slowing down (my guess) but that does sound like it could factor into your problem :hsdunno:

The possibility of the blades themselves getting hot and gripping weird/too much comes to mind also but I'd expect that to show up sooner than 20 minutes.

The spring thing, as was suggested, does make a lot of sense but, again, I wouldn't expect it to show up only after 20 minutes

I'm going to assume the rainfall is pretty much the same when you begin to drive as when they start acting up - this leads me to guess that because they slow down as the motor "fatigues" that the blades are then able to grab hold of the windshield and then you get the skipping.

I don't have this problem myself and I use cheap blades but I also don't often drive in the rain for long periods of time.
I would try either getting a diff. brand or buying the whole assembly instead of just the blade refills (if that's what you're doing) and maybe that will help.

If not, when you notice the skipping, what speed are they on?

If they aren't on the fastest setting you could try that to see if it clears up - it may be counterintuitive to go to a higher speed if the motor seems to be getting tired but it may help and better to have to replace the motor sooner than get in an accident

P.S. - I've always bought the whole assembly together instead of the refills myself just fyi, where I get them it's normally the same price or damn close to it.