Rambo actually played about 25 Tests under Khan, many of those were in a period when the team was going through a transition. Besides, he has couple of hundred & he played decent in away Tests. It was his last 30 or so Test, many played at home that he was really poor, probably averaging below 30.
One interesting point is under Imran, he payed mostly when Mohsin Khan went to Bollywood at his prime & Qasim Umar left the team, which actually makes sense, because Mudassar also retiered after 1987 UK tour - apart from Javed & may be Malik & Shoiab,hardly any batsman in that period of 1986 to 1991; therefore Rambo had to fight it out with the likes of Mainul Atiq, Rizwanuzzaman, Mansoor Akhter, Haroon Rashid, Asif Muztaba & few I have forgotten names. Funny part is, he played over 25 Tests at <30 average from 1992, when PAK had Saeed, Sohail, Inzi, Basit, Ijaz - on top of Javed & Malik; therefore it must not be Imran's fault.
He was a far better ODI player than his stats, considering the then context. Average around 34-35 @ 60 SR & 7/7 hundreds, till 1992 WC Final, which was quite good those days.