Is PartyPoker rigged?

I very much doubt it, in the sense that there's any intentional cheating going on to make players lose.

In short, No.

However, there does seem to be something odd going on with it's random number generation.

• if you flop a high pair, you can expect someone else to have one too. AA, KK and 88 flopping simultaneously is rather common
• When multi-tabling, you can get a lot of instances where the card numbers are the same in several tables, although often of a different house.

To me, this tells me that there's probably something a bit off about the random number generation. Most random number generators aren't that random, after all. 

It's really no big deal - it doesn't change the game one bit - but it is something to keep in mind, especially if you get a high pair.