Hi David,
We had a PIX515E running site to site VPN and had a couple of snags with it, until we reduced the MTU using;
sysopt connection tcpmss 1300
This was suggested by TAC ages and ages ago, so we've let it run without any problems. The same PIX also runs RA VPN with no probs.
You can also enter this command in the ASA. Try using this command on both PIX 506 and ASA but start with a value of 1380, then down to maybe 1320 and finally 1300.
Don't bother with the 877 MTU size for now, leave them as default 1500 and see how you get along.
HTH Ali