instruments for GMM

[delcredere]

I have used the lags of the exogenous variables as instruments but get 'near singular matrix' error. What should I do pls. What should the instruments be. I am using GMM estimation.

Gdp=c(1) +c(2)*D(gdp)+c(3)*D(D(lab))+c(4)*D(D(londsv))+c(5)*D(D(capflow))+c(6)*D(res)+c(7)*D(D(hcap))+c(8)*inf+c(9)*D(gcap)+c(10)*D(domabsop)

Londsv=c(11)+c(12)*gdp+c(13)*D(D(logeds))+c(14)*D(exr)+c(15)*D(D(londsv))+c(16)*tot+c(17)*fis+c(18)*cred+c(19)*govsize+c(20)*dom

D(capflow)= c(21)+c(22)*D(capflow)+c(23)*gdp+c(24)*D(logeds)+c(25)*D(exr)+c(26)*rir+c(27)*open+c(28)*D(capgdp)+c(29)*D(capdebt)