oil temp sender help please...

[MaSnaka]

I have Stewart Warner guages and have been having trouble with a oil temp sender. Originally with a stock pan (390fe) the sender was installed into it's bung and I could read the temps on the gauge. I upgraded to a new pan (larger capacity) and installed a bung for the temp sender and the original sender never worked. I replaced that sender with a universal sender purchased through Summit and was getting readings again but they were all over the place, I could be sitting at a light idling and the reading would creep up to near 300F and then creep back down as I increase rpms and drive away. The readings would get pretty high with just normal driving. Anyway I purchased a new SW sender that is rated for the temps on the gauge (100-320) and I get nothing, put the universal one in while waiting for the replacement and it works, replacement comes and nothing. Anybody have an idea why? Should I keep returning the non functioning senders until one works? Is it normal to get bad senders? I ground the sender wire and the needle goes to the max. Thanks, John