last week i replaced seal and problem is gone but i have another issue....

transmission shifting great , doesnt matter cold or warm but every week trans will start having problem with shifting , i mean sometimes shift up then shift down in unnatural way and rpm goes high up to for example 4000 with no reason..... but after 10 minutes everything is normal for 6-7 days , then after 7 days problem will appear again. its 100% transmission

Well , when i change transmission(used one) i replaced only 5 litres of oil and i didnt change filter. i guess there is still orginal filter , i didnt open transmission. And now.... Its possiblle that old filter getting clogged sometimes ? or just valve body but question is how its possible that dirt will enter valve body if there is filter to stop dirt ? there is another way dirty oil can enter valve body ? its possible that new oil wash out old dirt from transsmision ?