red, itchy rash: knees, elbows, ankles, and now shins

I didn't find anything answering this question in prior posts, but I apologize if I'm duplicating previous discussions.

I have a red, itchy rash that comes and goes.  My first eruption was the worst, about 11 years ago when I was 26.  The bumps were on nearly all "pressure points" - all the knuckles, my wrist bone, elbows, even some on the skin over my hip bone.  It was horribly itchy and went away in a couple weeks.  Since then it's gone more than it's present but when I have it, it can be very itchy and maddening!  It's not constantly itchy, just sometimes.  Other than the initial outbreak, it usually appears on a knee or elbow and often then appears on the other side, but not always.  The addition of it spreading down my leg and appearing on the outside aspect of both ankles was new a few months ago, and now I have a mild outbreak that is only on my shins.

My dermatologist biopsied me for psoriasis (I have undiagnosed pain in my thoracic spine and SI joint area, and less painful in my hands, ankles, and feet; this plus the rash made my doctor suspicious of psoriatic arthritis) and it came back negative; I have a copy of that report if the terminology is useful.  I had a blood test for celiac (suspecting the rash might be dermatitis herpetiformis) but that came back negative (although I didn't see it myself.)  

I do have Hashimoto's hypothyroiditis, well controlled with Synthroid since diagnosis nearly 7 years ago.  I am generally healthy - good weight, regular exerciser, good job, great relationship, low stress, no sleep problems.  No history of eczema or any other skin condition, no known contact allergies except for nickel in jewelry.  No known drug allergies except a couple antibiotics.  No known food allergies/intolerance.

Can anyone offer suggestions for me?  I do use a powerful steroid cream (clobetasol propionate .05% - one brand name is Temovate) which seems to produce some results, but I'd like to know what this rash is!