I find it interesting that in every election since 1980, the more experienced candidate (by your definition of amount of time serving) lost. You mentioned in the OP that it happened 1988, 2000, and 2008, but notice that it also held true in 1980, 1984, 1992, 1996, and 2004.
Let's go back further:
1976: - Ford: 27 years (~2.5 as President, ~.5 as VP, 24 in the House)
- Carter: 8 years (4 as Governor, 4 as State Senator
1972:- Nixon: 18 years (4 as President, 8 as VP, 2 as Senator, 4 in the House)
- McGovern: 14 years (10 as Senator, 4 in the House)
1968:- Nixon: 14 years (8 as VP, 2 as Senator, 4 in the House)
- Humphrey: 24 years (5 as VP, 16 as Senator, 3 as Mayor)
- Wallace: 4 years (as Governor)[/i]
1964:- Johnson: 28 years (1 as President, 3 as VP, 12 as Senator, 12 in the House)
- Goldwater: 12 years (as Senator)
1960:- Kennedy: 14 years (8 as Senator, 6 in the House)
- Nixon: 14 years (8 as VP, 2 as Senator, 4 in the House)
1956:- Eisenhower: 4 years (as President)
- Stevenson: 4 years (as Governor)
1952:- Eisenhower: 0 years
- Stevenson: 4 years (as Governor)
So, not counting third parties, the least experienced (by this definition) candidate won in every post-FDR Presidential election except 1956, 1960, 1964, and 1972. '64 and '72 can be explained by the fact that the loser was a particularly bad candidate. '56 was a rematch between an incumbent and the loser from four years before. And '60 was between two equally experienced candidates, but I'd give Nixon's experience as VP more weight, so you could say that Nixon was the more experienced of the two.
So if Romney loses this November, we can honestly draw comparisons between him and the likes of McGovern and Goldwater as far as epic failures go.
