Digital BW, The Print

I've bought a set of refillable cartridges for UT7 ink from MIS. I'm 
using it in an Epson 2200. Things were working well for a couple of 
months but all of a sudden I'm getting air bubbles (foam?) in 
several cartridges.  It happened after I refilled them but I didn't 
do it any differently than I'd done several times before without 
prior incident.  Nozzle checks indicate significant problems in 
several cartridges. Purging the offending carts helps sometimes but 
the problems come back almost immediately.  Tapping them to try to 
dilodge the bubbles helps a little but again, the problem comes back.

Are there problems with these cartridges after a couple of months? 
Are you supposed to replace them after maybe 3 refillings?  Does 
www.inksupply.com still sell them (I couldn't find them on their web 
site)?  

I love the system but the printer is down. Anybody know anything 
about this?

Thanks for any advice.

Elizabeth