Having trouble dollar cost averaging up.

One of my first stock purchases was $Costco (COST.US)$ , I love Costco, it’s one of my favorite places to be. As stocks have gone up and down, I’ve had no problems dollar cost averaging down, bought more ford while it was down, more $Advanced Micro Devices (AMD.US)$ , more Starbucks, it’s all well and good. But Costco has only gone up, it’s my biggest gaining position, so I never bought more. The result is now my Costco position is a small % of my portfolio, but I can’t get over this mental hurdle of buying it when it’s up. I’m up 46% on it, and can’t convince myself to buy more, even though I do think it’s going to go up.
Anyone else experience this? Have any advice to get over that mental hurdle, or confirm that I shouldn’t?