Hello friend. My first guess without looking at your configuration is that you probably need to implement NAT translations on your Vlans that are trying to get out. If the VLans can talk to each other than it sounds like your intervlan routing is fine. The problem you're having is that your gateway doesn't know how to return traffic.
Try putting in an overload NAT translation so your Vlan clients nat to a static IP on your Vlan1 when they try to route through your outside interface.
assuming your on a serial WIC for your internet connection:
Serial interface should have:
nat outside
Inside fastethernet interface should have:
nat inside
Now you'll need to first make an access list to identify traffic you need to NAT
Something like:
ip access-list extended NATVlan20
permit ip 192.168.20.0 0.0.0.255 any
This will tag any traffic from VLan20 assuming it's using 192.168.20.0 as it's subnet (change ACL as needed to match your addressing scheme)
Now to create the overloaded NAT
First create a NAT pool with the overloaded static IP you want to use. I would suggest a different NAT pool for each Vlan to help monitor which vlan is sending traffic as well helping with any policies you want to put in later for those vlans. You can overload all of your vlans under one IP but you'll be limited later if you want to put in any policies for those networks (your call here).
So overloaded NAT:
ip nat pool VLan20Pool 192.168.1.50 192.168.1.50 prefix-length 24
So here I have created a nat pool with a range of 50 to 50 on 192.168.1.0/24 network. We're going to use the host address of 50 on our native VLan's subnet as the address we're going to NAT to for traffic traversing from VLan20 out our serial interface on our router. Next we need to tell the router how to apply that NAT. We're going to tell it to apply it to traffic coming from inside that matches the ACL we created earlier.
Simply done that is like this:
ip nat inside source list NATVlan20 pool VLan20Pool overload
Here we tell the router to overload (PAT) all traffic from inside going out that matches the ACL we created earlier to the address range of the VLan20Pool which is 50 to 50.
If your problem is that your ISP is receiving a source address that it doesn't know how to get back to, then by NAT'ing (PAT'ing) traffic on your VLANs going out to a static IP from your network that is working, should fix the problem.
It's a shot in the dark, but I've got a sneaking suspicion that is what you're running into.
