He averages 35 in 35 innings, that's fine. The key to him being successful is getting past about 20. After a turbulant start, he really converts into big scores. 4 100s and 6 50s in 35 innings is very good.
The series in which he has done poorly were:
-South Africa in India, 1999/00
-India in England, 2001/02
Since then, he averaged 41 (England in India), 53 (India in West Indies), 30 (India in South Africa) and 69 in the tour of Bangladesh. Seems fine to me.
After that tour of England (yes I know it was to England and that is significent), he has averaged 45.
Whether he can play well in English conditions remains to be seen.
In the First Test of the tour to England a few years back, he got bowled by Hoggard on 1 but scored 53 off 79 in the second innings. In the Second Test, bowled by Hoggard on 0 in the first innings and lbw to Flintoff on 5 in the second. He was then dropped.
Now, the fact he got bowled and LBW shows he does not enjoy English swinging conditions. I am adament that Sehwag travels with the Indian test team in case Jaffer is 'found out' so to speak in England or Karthik has a bad run of form.