1987 GXL (Mostly Stock, engine replacement abouot 30-50,000mi ago)

Perhaps some one may be able to tell me what too look at. It seems that my clutch is slipping at certain times. However it slips differently every time. It slips sometimes when it's almost freezing outside, but not cold enough to initiate a cold start response from the car... The slipping usually does not occur when the car is warm (normal operating temperature). It will slip when shifting from 1st to 2nd, or from 2nd to 3rd, but never continually or consistently. I'm not redline shifting when this occurs, I am not abusing the engine. I have recently replaced my tranny flulid to take care of certain transmission problems (tripping gear when shifting to 2nd)... I have recently replaced my stock rubber clutch line to a mazdatrix steel braided clutch line, also have adjusted my clutch pedal to engage the clutch quickly and reduce free play (as the tripping was resolved with that).

I'm thinking I may need to replace my clutch and spring, but I would like to know if there's anything else I should check out before I do that. Any ideas?