I have a WASR with about 4,000 rounds through it and only 1 FTE using the cheapest, crappiest ammo I could find.
First, get all the cosmoline out of it. Strip it (and the mags) and soak the hell out of it in heavy duty degreaser from the hardware store (cheaper than blowing through 4 cans of Break-Free). Blow out all the nooks and crannies with compressed air, wipe down and reassy.
Second, function check it. Without a mag, cycle the action about 50-100 times. Check for any binding or unusual action. Pop it open and look for metal shavings or wear spots. If anything seems out of place, it may just need some Dremel TLC in a few high spots.
Third, check the mags. Mine came from the factory with a dented mag, so I couldn't get past 5 rounds without it failing. After stripping it and pressing out the dent from the inside (check it with calipers though), it worked perfectly.
Fourth, dry run with a mag. Insert and empty mag and cycle the action about 30-50 times. Check for any dragging or excessive movement. Eject the mag and look at the feed lips. Any wear on it may need a light buffing. Do the same for all the mags you have.
Fifth, cycle the mags without firing. IN A SAFE PLACE, insert a loaded mag (or use snap caps if you happen to have 30 of them laying around) and cycle the action 30 times. DO NOT PULL THE TRIGGER! Check all the above mentioned areas as well as the individual rounds. You should see some light striations on the casing, but any gouges or dents are a bad sign.
If it passed all that (takes about 2 hours for the degreasing, 30 minutes for the rest), it should fire without a hitch. I've had mine at the range when others have had a few problems, wondering why mine worked so well. We ran through those steps together and there wasn't one we couldn't readily fix or ID the problem with.
These things are a workhorse that beg to be beaten. My only suggestion is to test and inspect it before putting rounds down range. No sense riding a horse in to battle if it only has 3 legs.
On a side note, TAPCO mags are really hit or miss. I'm not a fan of the plastic feed lips and it seems that more and more plastic gets chewed off every couple of times they're loaded and SEEM to contribute to failures. Spend a few extra bucks and get metal mags. Even the worst surplus mag was able to be made near-perfect with a little time and TLC. (I have a few TAPCO's if you want to trade though

)
If you don't take care of your gear, don't expect it to take care of you.